A "gentle reminder" back that we are talking about someone who stopped attending work, became depressed and who has found that they actually couldn't live with their girlfriend.
You cannot judge an autistic 20 year old by NT standards. You have no idea how significantly he struggled living away from home, though it certainly sounds as though he realised that he was not ready. The poor sleep patterns, wandering around at night, etc are very typical.
OP I don't think your DP understands autism. It's really important that your DS understands that he has a home with you as long as necessary. There will be certain compromises, such as using headphones when he needs loud music, that he should be encouraged to make.
On the other hand, I don't think it's unreasonable for your SS to want/need his own space, and it is definitely inappropriate for your DD to share with either boy.
First, look at bedrooms. If you and DP have the largest, swap with the boys (assuming your bed fits in their room). Your daughter should have the smallest. Give the boys the biggest room, making it clear it is temporary until one moves out or on.
Then I would sit them down, talk through the problems from when they shared before, and see how they propose solving it. They might decide to store all their stuff in the room in separate spaces, and take it in turns a week each to sleep in the room and on the sofa. They might decide to screen or curtain the room. You must have rules about SS not touching DS's stuff and vice versa, and no loud music - use headphones. Basically, have a guided conversation with their ideas on how to solve this problem. Having the largest room might make dividing with bookcases or similar easier.
Your DP has to stop making DS feel unwelcome in his home. I can't stress this enough. He tried moving out, and it didn't work. That's humiliating enough without getting a constant message that you should not be here. He's only 20, he isn't a 45 year old man FFS. If SS goes to uni, would DP like it if you instantly return the room to DS and tell SS he shouldn't be there when he comes home after the first year? No? Then he needs to pipe down.