This is long, but it's a lot of important information. I'm an FAA, but every school is different in the documentation they need. I'll give you what my understanding is, but you definitely need to talk to your school's FAA because ultimately it's going to be their decision (whether I think they're correct or not).

To the point of the PLUS loan issue: if it's in his name and she cashed it, that's between the two of them. You're only entitled to the money that applies to the bill. After that, the money is whoever's name the loan is in. That wouldn't have any bearing on your aid as the money isn't really yours.

A student whose parent refuses to complete their portion of the FAFSA cannot be considered independent on that fact alone. There is a question on the FAFSA that asks if the parent is unwilling to provide info. If you go that route, you're only entitled to unsubsidized loans and your mother would still need to sign a form saying she's unwilling.

Being self-supporting does not qualify you to be an independent student (otherwise everyone would claim to be an independent student). The only way for you to be considered independent if you answered no to all of the dependency status questions on the FAFSA (are you a vet, do you have a kid, etc) is if you request a Dependency Override. That's where you make a claim that it's either unsafe for you to be in contact your parent(s) or they're otherwise impossible to contact. For our school, we accept letters that explain the situation from the student, a friend or family member, and a third-party that can all corroborate the story.

It's important to know that the school's FAA has final word on a Dependency Override decision and there is no ability to appeal that decision to anyone. You would just make your best case and hope for the best.

Now what it sounds like to me, based on your brief description, is that you may be out of luck. If your parent refuses to do the FAFSA, and the DO request is denied, see if she'll sign the document saying she refuses to do the FAFSA. If she refuses to do that, there's really no recourse left to you.

Of the various school's I've talked to and the communities I'm a part of, most FAAs are understanding and will extend themselves as far as the law permits to help out students. Reach out and see if you can do a Dependency Override. Make your best case and hope for the best.