Okay... further rambling about the syscourse stances of characters, let me just go back to the Jace Beleren thing.
Because Jace is... a literal mind reader.
That's a complete game changer in terms of headcanons that you REALLY need to account for. I mean, if a telepath met us in person, they would literally hear multiple voices conversing in our thoughts, each with their own "tones". That seems... really hard to deny.
I don't see a mind reader being even sysmed-leaning if they ever so much as met an endogenic systems. And if they hadn't met one... where are they getting their information?
I don't think most of the Planes in Magic: The Gathering's multiverse have "psychiatry" in the way we do. If anything, I think worlds in that multiverse that had a concept of multiplicity would primarily have "sysmags" who believed all multiplicity came from magic. Especially since many worlds do have possession states by literal spirits, and shifting personality states from curses and the like.
So does this scenario assume Jace Planeswalks to Earth and hears about it on Earth's internet without ever meeting an endogenic system in person so he could hear their thoughts?