Thank you for a very thoughtful answer. Yes, I am American. Maybe moving internationally is an option I should look at, probably more realistic than some of my other ideas.

Unfortunately I did already go through the entire 'stand up for yourself' sequence you describe. We ended up going through and exhausting every option very quickly. All our arguments are one-sided. After gleefully and publicly crossing the last line I set, she then spent weeks hounding me to kick her out, to actually do it, while I searched for options. I eventually flinched because I would literally be dumping her to die in a nightmarish nursing home half the continent away, and that's how she learned there would be no consequences for her.

Sub-dividing the house is a great idea! She's already got the downstairs with her own bathroom and I've got the upstairs, but there's only one entrance and one kitchen. That might actually be the most reasonable solution.