You've said that all her relatives know what she's like and expect her to behave badly.
She went to her parents and had to sleep on cushions Boo Double Hoo, that's hardly depravation.
Why are you feeling guilty?
Her parents are dealing with her. You tried to help but she couldn't accept the conditions of her stay, and then had to go to her parents.
This is NOT on you.
Her behaviour is what she's always like, it is NOT your responsibility. In fact I bet everyone was expecting something like this.
She's having a "Look at me.. poor victim" flounce and it's working on all of you.
Yes time is precious but she has managed to see them all for a while, even if not the full three weeks. And it is not your fault that SHE CHOSE to stay with a friend for part of those three weeks. Perhaps a full 3 weeks was too ambitious all round and her current absence means that there will be less confrontation.
Why do you feel guilt when you are not her relative and you have absolutely no control over what this "very tricky person... Very aggressive, confrontational, awkward, selfish, bullying at the best of times" adult person chooses to do?
Don't expect an apology, you won't get one. You offered your help. She took it and then you had to ask her to leave. That's on her. You've made it clear she could come back but she would rather have a little strop.
Take a giant step back. None of this is your problem. You did what you could. Let her do what she feels she has to do and let her family deal with her. She got to see everyone in the end which was the main object. Its not on you if she can't deal with people for 3 weeks.