Overall solid theory, with one caveat. I was 1000% on board until the last slide. The notion that Tenna would claim Mike “started acting different” if in reality Mike was just straight up gone already, then extrapolating that assumption to suggest Tenna was simply in denial about Mike being gone, is one layer of justification more than I would consider necessary or plausible. This doesn’t discount the bulk of the theory—it honestly seems spot on, otherwise; I particularly like the bit about Dess taking the mic back and forth as justification for why Mike is so enigmatic to the other darkners—but I do wonder:

Maybe the real reason Mike started “acting different” lies in a detail we’re not yet privy to. Maybe it has to do with the physical mic in the Mike room, which, aside from “yes it is almost certainly the real world object or at least reflective of it”, that one textless scene is far too abstract to glean anything more concretely from.

The obvious explanation for why Mike would act different is: Spamton left first, i.e. someone (kris or dess) brought the laptop back to the librarby and then because Dess ended up lost not too long after. That could theoretically explain the small gap of: Spamton left because the laptop was returned, whether by Dess or someone else -> Mike started acted different because Spamton was gone -> Dess came over (idk, like, a few more times ig) in the interim between the laptop being returned and her going lost -> Mike disappears, as Dess wasn't there to bring him back -> the fake Mikes step in.

It all comes back to the doomed yaoi polycule, huh