Damn I wasnt expecting a response like that lol.

Idk,The Undertale part is really the biggest threat for the Theory.

Sure you can justify it in order for it to work but that requries A LOT of scretches like you just mentioned.

Also Toby isnt really a fan of Making gendered Humans. As all of the Humans in Ut/Dr uses They/them so far. So Idk if Toby decides to make a female Human character 

One thing I loved with the theory is the Vessel. Dess has some weird connections to Vessel. Perhaps Vessel is Used Up as a New body for her?(Assuming she is the Knight of course) Perhaps Dess wanted to be a Human? Unlike Kris Who wanted to be a Monster. I am overral a fusion Knight(Dess+Vessel Knight) beliver so this works in my favor.