In attempting to answer your question about dogmas, I would begin by pointing out the obvious: namely, that every religious system, whether of the East or the West, is—as Schuon said—”an error compared to the Truth”. The religions are finally no more than upāyas, that is, “skillful means” or “saving mirages”, the soteriological point of which is to serve as pointers or portals leading us in the direction of the ineffable Principle. We are therefore best advised to look “along” these systems rather than “at” them. In a Christian context, one thinks of the resurrected Christ’s words to Thomas and Mary Magdalene in John 20: Thomas is invited to touch Him, a touch that leads to the most exalted profession of faith in the New Testament, “My Lord and my God”, while Mary is admonished, “Do not cling to me, for I have not yet ascended to my God and your God.”
The two Christian dogmas you mention (Incarnation and Trinity) point toward the paradoxical fact, embedded in these back-to-back exchanges, that Jesus both-is-and-isn’t God and, indeed, that God both-is-and-isn’t God. As I emphasize when teaching my university theology class, in rejecting various Christological heresies, the early Church was following a largely apophatic trajectory, insisting less on what ought to be believed and more on what ought not to be believed: one must not believe that Christ was only human (“adoptionism”), or only divine (“monophysitism”), or neither human nor divine (“Arianism”), or a composite of the human and divine (“Nestorianism”). Needless to say, the dogma of the Trinity comprises a similar koan, one which helpfully frustrates our tendency to think of God as Some Thing.
You ask, “What kind of belief is necessary for the spiritual Way?” Given the foregoing observations, I would answer by saying: a belief that allows one to “ride the crest” of these paradoxes. We aren’t really Christians if we don’t take what the Church teaches seriously; like Thomas, we’re obliged to confess, “My Lord and my God”. But at the same time we need not, and ought not, to become fixated on the formulations as such; like Mary, we must avoid all “clinging” so as to keep our minds and hearts open to That which transcends every form. “God is supra-non-knowable,” said St Maximos the Confessor—who, let us immediately add, was tortured and maimed for his uncompromising insistence on the Christological dogma of dyotheletism—”and He can therefore be known only by an act of supra-non-knowing.”
Perhaps I should append one other note. When you speak of an “assent of the will”, it sounds as though you have in mind a sort of “willing suspension of disbelief”—in other words, a purely volitional capitulation to a claim one would otherwise have every reason for doubting! I think of Tertullian’s fideist slogan: Certum est quia impossibile est. It seems to me, however, that both of the dogmas in question make eminent metaphysical sense, notwithstanding their koanic character. Indeed, it’s hard to imagine verbal formulations that could come any closer than these to expressing The Inexpressible. I’m tempted to go even further: it seems to me that what you call the “exoteric side of Christianity” actually does a better job of “mapping” the multi-dimensionality of the advaitic Self onto the plane of words than either of the other two religions you cite, Islam and Buddhism. This of course is why Schuon could speak of Christianity as an “eso-exoterism”, and why he said that “Christianity is not a priori a religion but a mystical brotherhood become a religion”.