I get what you’re saying, but I simply don’t see how God’s triunity excludes a subunity of corporeal incarnations, or vice versa. I’m not saying that the Son really did or would incarnate in several locations to several species, but rather that I don’t think the reasoning here against it is thus far sound.
Jesus did not exist before time. The Son existed before time, and as you say, He did speak to our redemption, which would be won through his incarnation as Jesus, but I still fail to see how this excludes his speaking to the redemption of another hypothetically fallen species. Everything He tells us pertains to us and our relationship with Him, but it does not exclude the possibility of another relationship. Certainly, it’s not explicit either way, but I would like to understand the implicitude in what was told to us, hence my asking in the first place. I don’t think anything about this hypothetical implies God lying about His true nature.