Purely my take on all this, but I see Thomas and Dominic as both mentally ill. Thomas's illness has manifested in this paranoia/bi-polar stuff that leaves him in such a state that he is a danger to himself (though I don't sense he'd ever purposely hurt another) and is better off institutionalized because caring for him would literally be a full time job. I don't know a lot about mental illness, so I'm spitballing here, but Thomas's situation seems to be the type of illness you're kind of "born with" - you know, brain chemistry, etc, and it feels like living with an aggressive, judgmental asshole like Ray exacerbated his illness.
Dominic, on the other hand, doesn't seem to have the paranoia issues that Thomas has, but he grew up walking on eggshells around his abusive step-father and at a young age, took on a lot of responsibility for his clearly unwell brother. I don't know what the DSM would say about acquired/learned rage issues in regards to mental illness, but Dominic never learned about how to manage anger, grief, disappointment. It was either duck and cover or fight back - he doesn't understand nuance when things get bad. So when Dr. Patel told Dominic that she sees two boys in the woods and only one stands a chance of making it out, she sees potential that Dominic can work on his issues and become mentally and emotionally healthy;Thomas is a lost cause. I think that the twins are the same in that neither of them would intentionally hurt anyone else; Thomas self-harms and Dominic is verbally abusive and takes his rage out on objects, ultimately to his own detriment.
If Dominic can overcome the damage of his prior life - the abusive upbringing, taking responsibility for his ill brother, the death of his daughter and the fallout from that - I think he'll actually turn out to be a decent person. The way he handled Ralph Drinkwater's revelation that Thomas had been abused in the facility surprised me, but it also impressed me a little. He was single minded in getting his brother out of there, so I can't disagree with him taking Thomas on (I mean, if it was my family member, you bet your ass he'd be coming home with me rather than leave him in there one moment longer than necessary). Consider that he walked out of the attorney's office when the lawyer started talking about a class action lawsuit - Dominic would have no financial problems in that scenario, and it would have been an easy win. I think a lot of people would have gone down that road. I figure Dominic knows that living with Thomas indefinitely isn't feasible, but all he knew was his brother was being abused and he needed to fix it; I understand that.
I hope that when the dust settles, that Dominic informs Rosie O'Donnell of why he did what he did. He's in a tough situation; Ralph Drinkwater did him a favor and he owes Ralph for that, including protecting his identity as a whistleblower. But there is he fact that other patients are going to be vulnerable to the same situation.
Finally, Domenico - yikes. As ugly as Thomas and Dominic's stories have been, Domenico's was even darker. I don't think I'd have gotten very far in that story if it had been my ancestor, but Dr. Patel was right - Dominic is learning a lot about himself and his family via that story.
Every week I'm impressed by this show. I have to put myself in the right mindset to watch it, and we have something much lighter queued up to watch afterwords as a palate cleanser, but it's really been excellent on just about every level.