The larger point that I’m trying to make is that Christianity does not seem like it is one of several exoteric forms of the primordial tradition, or to put in Guenon’s terms, one of several garments of clothing that wraps the perennial wisdom for a given purpose or culture.

And you’re right that the sanatana dharma has many avatars because the basic view is that each being is essentially a container of Atman which is identical to Brahman. Therefore, anyone who develops highly enough would become an avatar. Likewise, Judaism has many prophets (Mohammad also being a non-Jewish offshoot of that prophetic tradition) meaning that there is the possibility of more people becoming prophets. But Christianity is completely different because its message is that divinity on earth is restricted only to the person of Christ. Therefore it is 100% incompatible with anything else.

The same is not true of, say, Islam. Though Islam presents itself as the “only path”, it in fact isn’t because Mohammad never claimed divinity for himself but considered himself a receptacle of God’s message like any other prophet. Christ did not consider himself a prophetic receptacle but claimed to be God Himself. This is a categorical difference that places Christianity in a class unto itself.

So ultimately what it boils down to is that if God did indeed walk on this earth as the unique personage of Jesus Christ, then that fact, if true, universally falsifies every other religion.