These questions certainly are difficult. I have a few thoughts.
One thing that kept on popping into my mind, is that it is near impossible, or perhaps actually impossible to control an adult child...and with a Difficult Child child, well the odds are worse!
A friend of mind...her daughter didn't send thank you cards after her wedding until perhaps ten months later. My husband said something like "Well, her mother should of insisted!" Hmmm. Say what? This girl has mild Difficult Child tendencies. I said it didn't matter what mom did...the chances were extraordinarily high that this girl was going to do whatever she chose to do.
I think you have the right to set up terms and boundaries. But, maybe you have to take into account his abilities (like you did by lowering the rent greatly) Additionally, you probably shouldn't become overly attached to the outcome.
We have found setting boundaries with our Difficult Child is usually a good idea. But, we have also found that we can't set up too many and do have to let some things "go." Additionally, some take longer to take effect...but consistency is key.
We have a boundary that Difficult Child can not call us before 7:30 a.m. during the week and 9 a.m. on the weekend. This one was easy, because it's our decision whether or not to answer the phone if she breaks the rule. She can text us if it is an emergency. It took a very long time for her to cooperate with this boundary. But, I would say today she is about 90% compliant.
I also would agree that there is no right or wrong answer with this stuff. Very few would fully understand what us parents have gone through. I find it near impossible to fully explain the complexities and oddities that involve my Difficult Child.
Just today, she "threatened" to go to the emergency room mental health unit because I wouldn't buy her a gym membership. I acted non chalant suggesting she contact her psychiatrist or psychologist...which she evenutually did and seemed to feel better.
by the way, if in fact he is going to group therapy...this might be a very good thing...especially if the group leader or group members can see through an BS and call him on it!
Hang in there!