I was actually waiting for you to post your theory on Discord first before I reply, but since it has been a few days, I probably should just reply here
I can't really say much because of spoilers and stuff, but I just wanna point out that the new villain isn't the main one, just another side antagonist like Kiyo. She isn't possessing Rin either, just brainwashed her (′v‵ )
Other than that, your theory is really fun to read haha, be sure to post it on Discord so other members can theorize! (-ω-ゞ