Yes you’re right - this is not a recent thing
I knew it wouldn't be
Your oh lack of support means she feels conflicted and confused - it's not helping her at all you both not being on the same page and certainly not his blowing hot and cold on the matter
His parenting sounds as if it leaves much to be desired!
If he doesn't want "she has to move out" to be a sanction then you must TOGETHER agree on others, tell her clearly what they are, and stick to them - it's the ONLY way to be effective.
She is an ADULT mh issues or not, in the near future she is going to have to manage her own home inc keeping it sanitary and tidy for health and safety reasons if nothing else. She's also going to have to manage her own finances.
Our job as parents isn't to rescue them from their flaws, but to support them to become capable, independent adults who can live alone and manage all that this requires.
Ideally this starts at a young age, I raised dd alone from she was a toddler and straight away she was expected to help out - at an age appropriate level and standard. So starting with putting her toys away and her dirty clothes in the hamper with me at the end of each day.
Then as she got older she gradually did more and more independently, quite honestly by the time she was 14 while emotionally she wouldn't have been ready, practically speaking she could have managed our home alone for between a week and a month
I did/do notice that other parents have not taken the same approach. The first time I really noticed this was as a mature student at uni for the second time. A large number of the 18/19 year old students were very unprepared for living away from home. They struggled with basics like laundry, cooking, budgeting... because their parents simply hadn't bothered to prepare them. I was alerted to one house share group having difficulties paying their rent (I was a student rep and so was often approached about this kind of thing) all the students in this house share were the younger end of the year and had mostly been privately educated. Turns out the reason they'd no money for rent was because they had been living on expensive (not even usual student type cheap) takeaways - and they hadn't even been clubbing together to get them so they'd each order a huge amount they couldn't possibly eat in one go and they weren't even saving the leftovers they were throwing them out that night - because nobody had taught them how to save/store/reheat leftovers. I helped them (at their request) create a sensible budget, I also made them a notebook of basic, cheap, easy recipes that they could batch cook and freeze or would feed the whole house in one go - they were even doing things like they each had their own bottle of ketchup, carton of milk etc rather than having a kitty and getting basics out of that. They'd also been shopping for basics out of the expensive corner shop rather than going to the supermarket - that was literally across the road! They knew nothing of yellow labels or seasonal shopping etc. I actually really enjoyed working with them on this and they got a kick out of finding bargains and leaning to cook and do laundry properly etc
I would strongly recommend assertiveness training for you both - incredibly useful skill I was taught as part of nurse training (first bout of uni) as at that time I was painfully shy and would have been a total walkover for aggressive staff and patients.
Perhaps also a parenting course? Or read up on parenting teens/young adults? I've done my share of that. Dd and I went through a difficult phase just before she moved out which is normal but can be very upsetting.
Speaking as someone with mh issues of my own (ocd, agoraphobia, depression, anxiety, phobias) clarity - especially clear boundaries REALLY helps. This is actually something I'm working on more myself with a therapist at the moment. Bit trickier as I'm having to both set myself boundaries AND enforce them - feels a bit "split personality" at times. Timetables help too - reasonable ones of course. "Sort this whole room out by tomorrow or else" is of course just setting her up to fail but "within the week I want all your stuff that doesn't belong there out of the living rooms and properly stored" reasonable.
Hoarding is possible (I struggle with this myself) as is ocd or similar conditions where control and order are highly desired. But there are ways of managing these there are workarounds.
Compromise to a degree too. My dd is a complete slob! Definitely not her mothers daughter 
but my rule was as long as it was limited to her room and there were no biological agents growing in there then whatever just keep the door closed - sounds simple but with my ocd it definitely wasn't!
What is the storage like in her room - does it work for how she is? Or is it to you and oh taste but not especially practical?
Eg dd used to drive me nuts with not hanging clothes up, it then emerged she couldn't hang them up (this was because of a disability she has which we didn't know at the time) because after she'd put 2 or 3 items on hangers and in wardrobe her wrists and elbows were hurting. So...got rid of wardrobe and got a big chest of drawers instead which she could manage more easily. Ditto the under bed storage she found it hard when it was full to pull it out/put it back so got her the kind on wheels which was much easier.
Find solutions where you can
Anxiety is causing her to blow up.
I agree - I do the same so does dd
That's the point at which relaxation/breathing/grounding exercises are useful as is one or both parties walking away - and for walking away have a hard rule of NOT following and pestering someone who is walking away - give them their space