At the risk of being accused of wafty, permissive, over-liberal parenting, and having read your update op, in which you say she struggles overall with motivation, I would say that her room is an outward reflection of her inner struggles.
Why did she drop out of uni?
And why is she falling out with people left, right and centre?
Why is she only able to work pt at 23 years?
I would want to investigate those issues alongside her with the help of a licensed psychologist.
Of course she could just be massively lazy and entitled and spoiled, or she could be suffering from some form of depression or executive malfunction or both.
Sorry if I have missed this but when you say her bf comes over, does he stay overnight in her room? I’m quite surprised that she isn’t too ashamed of the state of it to invite him over!
Personally I would try my hardest to come to some sort of compromise. I know you have already tried this but she is obviously demand avoidant for some reason.
Treat it like we are all adults in this together sharing a space rather than from the top down. I know this is hard when she acts so defensively. That indicates to me though that she is massively ashamed with herself that she can’t keep on top of it. Instead of demanding, try and ask what specifically is stopping her from doing it? Listen to her. Ask her if there is anything you can do to help? (I know you shouldn’t have to when she’s 23 years old but you have to deal with reality.)
Maybe suggest she listens to TOMM guided cleans or you sit in her room and body double while she does it. Or suggest you help her get it back on track with the help of an App and then come up with a maintenance plan.
I know lots of people on here will howl in despair and say that this is infantilising her but I am an adult who has difficulties keeping on top of cleaning (not to an unhygienic level but to a cluttery level) and I know quite a few adults the same and coming down hard on them just doesn’t work, nor does it work for me.
I would definitely insist on her taking out old food, takeaway containers and old mugs of coffee etc or else you will have some sort of insect or vermin problem. If she doesn’t comply then you have the right to go in there and do it yourself.
I think I would probably insist on taking a day’s salary from her too and putting part or all of it towards the cost of a cleaner or at least cleaning materials. Then she will be more invested in keeping it clean.