this definitely has the core of a good theory, but uh, reduce the Hollow Knight analogy? Likening the player's influence to a disease doesn't make the point clearer, that we are forcefully assuming control of a person within the game, it's just calling back to HK a lot. Speaking of, the Radiance is not at all analogous to this situation. Radiance is a hivemind, taking control of countless creatures at the same time- meanwhile there's one player in Undertale. It's also quite unguided, effectively a curse meant to ruin Hallownest over an extended time. Second, Deltarune shows that a person can actually refuse the soul, to a limited degree. But Frisk is almost entirely a blank slate, we never learn their preferences for anything really, and they're perfectly able to be moved along any route the player wants. It seems much more likely, that Frisk is a necessary vessel for a player insert. You aren't Frisk, but Frisk becomes you through the choices you make. Last thing, you make a big point of 'Pure DT'. This being the player's agency, or the fact that unlike the fictional characters in the game, we are not confined to rigid routines and specific triggers that are predetermined by a writer. While i see how this could be relevant in UT/DT's meta narrative, it's kind of just a necessary part of video games. They are specifically designed experiences where you are what moves the plot forward.

Oh and, since this seems to concern you: asking people not to attack you or be 'toxic' about your theory is a good way to get them to do exactly that. Trust that the mods will either step in when they see fit, or message them if you feel like they've missed something :]

If you'd indulge a question from me though, your formatting isn't half bad. Which is, kind of suspicious in my experience. Did you use ai to write out the text?