Hello and welcome to the family. I'm so sorry you're going through this and I've walked the same path with my 26 year old BPD daughter. It's terrible in every possible way.
My question is, how can involved parents handle the impossible situations we are put in?
The simple answer here is to stop being an involved parent, so you won't be put in those situations any longer.
I realize that your son is sick and he needs help. But he can only receive help if he's actually willing to make changes in his life. You can't force that on him and neither can a psychiatrist or anyone else. It is 100% up to him to choose a different path.
So if you can't do anything to change the path he's on, the next question is why fight it?
Again, he's sick and he needs to realize he needs help. But if you're trying to coddle him while he's abusing you, then he's learning that abusing mom gets me whatever I want. In other words, he sees it all as your fault....simply because you continue to take his abuse.
A better approach would be to stop fighting him, stop running in to save the day, and let him deal with the fallout in life himself. If he wants to be wild and free, no problem, show him the door and wish him luck. Because once he's on that path and can no longer blame mom/family, he will eventually come to the conclusion that maybe he does need to make a change in his life decisions.
This happened with my BPD daughter at 23 after she bounced from house to house for almost two years. She wasn't welcome at home unless she could help out and be respectful. She felt that was too much to ask and hey, that's no problem...she decides the rules for her life. I finally let her do her thing and face the consequences.
At 26, my kid is employed, paying her own bills, and very respectful to her entire family (including me). She spent two years in intense therapy and made so many huge changes to her perspective and how the treats others...she's almost not the same person anymore. And while she does still have terrible days from time to time, she knows how to respond in order for her family to support her in healthy ways.
Your kid has to make these decisions on his own, and at the same time you have to remove yourself so you're no longer a part of the problem. You desperately need that to actually heal and see this more objectively as well. I hope that helps.