OK, first of all, I think I can clarify our differences on this point, but also point out our agreements. Byleth is not Sothis's reincarnation. They have different souls and minds. We both agree on that point. However, I understand the argument you're making here. That Sothis moving to Byleth's body is her own reincarnation, separate from Byleth, but sharing the same body as him. I still disagree, but it's just a matter of opinion on the definition of reincarnation, which varies across religion and media and is not something to be argued about.
We both agree that the situation as Rhea/Seiros would have wanted it (Sothis alone in Byleth's body or at least completely dominating Byleth) WOULD have been Demonic Possession, but it's only theoretical. Rather than Demonic Possession or Reincarnation, perhaps the best tropes would be either Symbiotic Possession or Sharing a Body? Sothis's Crest was her Soul Jar, and then she got transferred to Byleth's body, creating a situation where she either shared the body with Byleth's soul (as the lesser aspect, being only able to sense, not command) or symbiotically possessed Byleth's body alongside his own soul (again in a lesser aspect, only able to sense). Eventually, they undergo Fusion Dance, at which point the Crest Stone becomes more or less irrelevant. They're still separate souls and minds, but so tightly linked they can basically read each other's minds.
I think we can both chalk this second one up to misunderstandings and awkward wordings. My complaint was purely with the entirely inaccurate initial version. If you thought otherwise, I must have phrased my thoughts badly.
And I think you're misunderstanding me on this last one as well. Marianne has every right to The Stoic trope. Once again, my problem is with the description wording as it currently is. It should be clarified, to the reader, that the trope is being played with as Marianne's stoicism is a facade constructed by her fellow students that hides her shrinking violet. Lacking that, it's incomplete and can give a reader the wrong impression of a character. So my problem isn't with the use of the trope, but that it isn't being properly expanded on. Right now, the description makes it sound as if Marianne The Stoic is being played completely straight, when we both know it's being played with.