There are two main charges against the Church of Jesus Christ of Latter-day Saints as it pertains to the claim of the title of Christian. The first is that Latter-day Saints aren't Christian because they don't accept the trinity doctrine as described in the various Catholic creeds - this one is mostly already addressed in the first essay on the subject, https://ldsdefense.com/essays/the-trinity/. The second is that they aren't Christian because they don't believe in a fully divine Christ, which this essay will be addressing.
Before grinding the rail of theological discord, it's important to note one massively glaring point in all of this debate: If LDS theology is correct, then the LDS conception of the Godhead and Christ's origin (if any) is correct. LDS theology doesn't need to sit comfortably with anyone if it's the truth as the truth is not reliant on comfort, let alone Greek mysticism or Islamic religious philosophy, both of which are used to create the conception of the creedalist Godhead and its (non-)origins. The primary purpose of apologetics, in this instance, is not to shape the LDS conception into something comfortable, although some of that will occur, it is to correct the claims such that those seeking the truth can arrive at it through less obscurity and more honesty. Let them discover and sit with the truthfulness as they may after that.
To complement the above point, another note worth considering is that when creedalists or others attack Latter-day Saints on the issue of being Christian or not, they almost universally do so through the assumption that their own theology is correct. This is begging the question in its most basic form - relying on their own contested theology to produce the definition they're using to exclude other theologies.
Now to the grind - there are numerous logical errors in discounting Latter-day Saints as Christians, but there are also likely purposeful, dishonest conditions necessary to arrive at the exclusion. The errors involve assumptions, assertions, and definitional smuggling regarding divinity and soteriology not relevant to the actual claim, and the conditions require not only straw-manning the theology of Latter-day Saints, but outright misrepresenting the same. When evaluated on their merits and intents it's clear that not only are Latter-day Saints Christians, but they arrive at Christianity unreliant on pagan or islamic philosophies, which are precisely the operative frameworks used to malign Latter-day Saints on the issue.
The Definition
If we are to define the title "Christian" by a somewhat objective, secularist lens, we might say something like:
"A Christian is a person who participates in religious beliefs centered on Jesus of Nazareth and regards him as a messiah or saviour."
Your first response to this might be repulsion at its genericity and that's understandable, but consider why that is. If it's because of who that might "let in" then you've already arrived at the purpose of this examination - to point out that using "Christian" to gatekeep your theology requires your theology to do that gatekeeping. In order to impose more criteria, you have to look at your own theology from which to gather them from, or the theology of whose traditions you feel comfortable sharing the title with. In order to be more specific, you must beg the question with your own theology.
There is no secular means to adjudicating the correctness of the title, there are traditions that claim it, identify by it, and each use their own definition of the title - it is by its very nature unfalsifiable and unverifiable as is any unscientific definition - the language is owned by those who use it. So there's no way to adjudicate that which is effectively opinion and even the generalist definition proposed could be considered too specific by some.
Thus, adding specificity doesn't achieve correctness, it achieves gatekeeping.
The Gatekeeping
For most creedalists who claim the divinity of Christ on the basis of His eternal nature and oneness of the father, they do so requiring several logical concessions. First, they state that God the Father and Christ are one, yet distinct, and provide no analog for this. They say you cannot call them separate, despite separation being the effective meaning of distinct, you cannot call them one being, despite that being the unavoidable consequence of naming three persons, and you cannot call the substance divisible, despite distinction requiring something to distinguish between. They've created a negative (apophatic), two sided cliff where if you try to make a positive (cataphatic) claim, you fall off on either side, usually into arianism or modalism.
Where this language and these requirements come from is particularly dubious for a group that is trying to gatekeep Christianity - they're derived directly from the requirements of the Greek mysticism they inherited. The creedalist/trinitarian tradition is built from the floor up on neoplatonism and aristotelianism and even includes Islamic religious philosophy - supplied on top of the aristotelian metaphysics that Aquinas hedged the theology up with. The notion of divine simplicity that forces them to reconcile three into one - that's Neoplatonism, a Greek mysticism. The technical philosophical apparatus Aquinas used to argue for that simplicity and unity - the essence-existence distinction, the necessary existence framework - that came from Avicenna, an Islamic philosopher.
There are many more examples of external, non-biblical frameworks applied to virtually every level of the creeds and their philosophies - they rely heavily on Greek mysticism as noted in other essays. The point to bringing them up is that the gatekeeping done by creedalists is done so largely through a Greek and Islamic lens, not a purely biblical one. Thus, to gatekeep Christianity using the tools of not only un-Christian, but unbiblical philosophies is clumsy when you're not necessarily even inside the gate.
The Lack of Concession
When creedalists make the attempt to exclude Latter-day Saints, they do something very peculiar with regards to their own tradition. They've built upon extra-biblical frameworks and logically incoherent concepts with no earthly analog, and then demanded concessions for them - that you assume the same thing they do, that the external frameworks are valid and that the invalid logic makes sense outside of our earthly epistemology. Those are not small concessions and they only ever escape scrutiny on the same basis that most Christians are unaware of just how deep their Greek theological lineage goes - because the traditions have been around so long that people don't seem to notice them anymore, they just seem intrinsic and rigorous via the weight of time and tradition.
It is of no serious concern for Latter-day Saints to grant those concessions, even in light of their grandiose nature - if earnest people trying to follow Christ want to consider themselves Christians for the betterment of self, others, and the earth, then it is by all means the practice and policy of Latter-day Saints to welcome them into the fold of Christianity. The problem is, the favor isn't returned, and the concessions required for Latter-day Saints are not nearly as large.
Latter-day Saints believe that Christ is fully divine, eternal, and begotten or as some LDS scriptures put it "organized". The scriptures arguably read this way, so little concession need be made, and the little concession that is necessary is to simply put in any effort to reconcile those potentially conflicting notions (if they are at all) and hold them provisionally true. This is a far cry from the concession that must be granted regarding the creedalist contradictions of distinctness and oneness. The LDS theology doesn't state how, when, or in what exact way Christ is created, and one could even presume their own mysticism to resolve it by assuming time outside of time, space outside of space, or even some "retroactive eternity". The point is not to assume the answers, but to point out that creedalists demand concessions to their extra-biblical theological manipulations and illogical apophaticisms, and then are unwilling to provide any concessions to Latter-day Saints of even a lesser degree. This is not only hypocritical, it's potentially outright bad-faith.
Another Greek Failure
So, if there's some potential interpretation for Christ to be eternal and without inception, then why don't Latter-day Saints err towards that end and save themselves the trouble of the debate?
The answer to this question has a few parts. First, Latter-day Saints actually believe that as a spirit, Christ really doesn't have a beginning, so missing this is part of the potential bad-faith seen from those who would straw man and misrepresent rather than steel man.
Second, the response to the first point may be that Latter-day Saints don't believe Christ to be fully divine from inception - that His divinity was some way earned or grown into. The problem with this criticism is that not only is LDS theology unclear on this point and making assertions for the Saints would be purposeful straw-manning, but there is no logical or biblical requirement that Christ be "fully divine" - whatever that is exactly to mean - from inception. It has no soteriological effect whatsoever for Christ to have not been "fully divine" at one point prior as long as He is now. Some have even made the case that achieving divinity is far more admirable than inheriting it, but the theology is genuinely unclear on this point so to make a hard distinction against the Saints that they don't make themselves on something that wouldn't affect your salvation in any way is once again straw-manning as well as non-sequitur.
Third, the Church of Jesus Christ of Latter-day Saints doesn't suppose itself to be a reformation hoping to achieve the right doctrine, it considers itself the true, restored gospel of Christ, directed by Christ Himself. Carefully devising theological vernacular to conform with others while you've got a direct line to Christ would be like listening carefully to someone on the street give advice about how to find a wife while standing next to your own - there's no need for careful consideration of wording regarding the nature of God from other men when God is actively revealing to you that information Himself.
Finally, the question of "divinity since inception" is not actually even covered in the bible - there are some verses this is inferred from, but the bible never explicitly states such a notion. The reason Latter-day Saints are unapologetic about their use of normal, linear English - echoing that of the bible - is because they are unreliant on the Greek mystical system that requires it. The Plato and Aristotle that creedalists inherit requires strongly that God be timeless, but Latter-day Saints don't inherit that mysticism - they can read the bible without having to constantly shape God into a single, amorphous substance. Thus, having no need to answer to others, no need to speak in a way congruent with Hellenes, and it having no effect on their salvation or the saving nature of Christ, Latter-day Saints can comfortably speak of Christ having had a beginning or something like it.
The Brother
The final attack on Latter-day Saints and their view on Christ's divinity is that Satan is his brother. The problem is that this is yet another straw man. Not only is satan not Christ's brother in any meaningful sense - LDS theology doesn't hold that they grew up together or wore the same clothes - but the theology is clear that Christ is uniquely the "Only Begotten" and that satan doesn't have a body. So you can't with a straight face read about the LDS tradition and conclude that they're related in a way that would in any way hinder Christ's divinity without suggesting that His divinity would be hindered by our own relationship to Christ.
There are numerous scriptures that state that we're all children of God - if we're all God's children and Christ is begotten of God, it would stand to reason that we are sisters and brothers with Christ. Once again, creedalists can't abide this logic because of its defiance with Hellenes, but even in Matthew Christ says "I ascend unto my Father, and your Father" - making it clear that we are brothers and sisters with Christ at least in some way. So while you can disagree and have a separate reading from Latter-day Saints on this topic, you can't suggest that the LDS conception isn't biblical and even in LDS theology satan is no closer related to Christ than we are.
Suggesting a relationship to satan would hinders Christ's divinity actually puts a far stronger leash on Christ's divinity than Latter-day Saints would suggest. The Saints can handle Christ being even a direct, blood brother to Satan precisely because of the level of divinity they attribute to Him and attempts by creedalists to suggest otherwise effectively suggest that Christ's divinity can be diminished or contaminated by a sufficiently evil association. This notion reduces the atonement from infinite to "just barely divine enough to mitigate the most evil human". If having some association with evil would effect Christ's divinity for you, then it's fair to say you don't actually think He's fully divine.
The other problem is that Christ had real, earthly brothers. If having an evil spirit as your brother is a problem, why is having a mortal, sinful brother not a similar problem? Even if your tradition rejects the idea that the brothers mentioned in the New Testament were literal brothers, your tradition likely accepts that He could have had brothers as Mary was mortal and there was no indication she couldn't bear more children.
Creedalists suggesting that Christ's divinity would be challenged by having some spirit relation to an evil person when they themselves believe that God also created the spirit who became satan is like telling your kids they can't be track stars because you have a distant uncle who was a chain smoker. There's no logical or biblical reason why being distantly spiritually related to anyone, even the most evil, would affect Christ's divinity. One of the most prominent aspects of LDS theology is agency and making such a suggestion would be in direct conflict with this principle.
The unknowingness of Creedalists.
One of the most important aspects missed in this debate is that those who are of or come from the catholic tradition that created the creeds inherit its philosophy, and one of the most prominent principles in catholic philosophy is that God is essentially unknowable. This is in part why creedalists sit so comfortable with these logical contradictions created to satisfy the external philosophies - because their tradition affirms vehemently that God cannot be understood and that you can say more about what God is not (apophasis) than you can about what He is (cataphasis). This is particularly relevant, because the creeds hedge up around what you can't say about God and in most cases don't deign to resolve their own conundrums on this basis. Compare that to the LDS conception of Christ and His origin (or lack thereof) and you'll note that the LDS conception unapologetically uses normal, linear English, but with no strong explanation for how any of God's workings actually function in the spirit or material worlds.
What this results in is one tradition's unbiblical, unfalsifiable opinion around not-knowingness gatekeeping against another tradition's not-knowingness that uses regular, biblical language with little explanation behind it. It's hard to imagine a comedic skit reaching this level of preposterousness.
Refusing Latter-day Saints the title of Christian on the basis of the divinity of Christ when the basis of truth is unfalsifiable, the claims against don't actually affect Christ's divinity, and the Saints themselves declare Christ's eternal divinity is not only absurd, but likely a dishonest attempt at leading people away from a more biblical, or at least less pagan faith.