I've made the assumption, possibly wrongly, that op understands PDA and has been using the strategies the PDA Society recommends with no success. I've also made the assumption that her therapy/ involvement in any local groups centre on PDA profiles of autism. It's an assumption because I don't think she has specifically said.
A more robust PDA friendly approach is definitely needed (which will still look very different from the strategies many posters here are calling for) but I think the op wants to try the tough love approach because she's spent 2.5 years in a frozen stasis taking a no demand/ gentle approach which hasn't worked so thinks it may resolve things. It might do of course, if this is simply a manipulative maladaptive 19yo we're talking about (personality) who is choosing to behave this way. And she might be.
However, at this point, and given everything op has said about her dd, the far more likely impact will be that she regresses further - perhaps to the point of no longer being able to do even the limited things she does now. This won't be a popular opinion though, and my intention isn't to worry the op more than she is already.
PDA is an incredibly difficult profile to understand and work with, so I totally understand why the 'toughen up and distance yourself' approach is being recommended and will most likely be pursued. Adults like to think that they can take control of situations and make others behave in a more acceptable way. It's human nature. And it often works with neurotypical younger children who can do well, by virtue that they have a brain that's wired for success in a neurotypical world. The trouble is when that child is no longer a child, but an adult with extremely inflexible thinking coupled with very high levels of anxiety/ mental health issues. It's really hard for parents to differentiate in this situation between what is a behaviour choice and what is a genuine inability to do differently.
I am prepared to be dismissed/ flamed for this and I know I'm in a minority of one on this thread, but I genuinely want to help op and can't just sit here and say nothing if saying it might resonate with her - even if further down the line if/ when hardening up to her dd doesn't work the way she wants it to. I genuinely hope the advice she's taken on board from other posters works for her.