This is my take on this situation.........ONLY.. because I have been there, done that. I am not going to type everything I went through with my son, but....... He was diagnoses with all the fun stuff...ADHD, ADD, ODD and was a complete horror for about 12 years. I sought help from every avenue I could think of. Wound up placing him in a facility that had severe restrictions and a padded room. Little man had a melt down he wound up in the "calm down" room with the door locked. He learned to control his anger and do more talking than throwing things.
It took a couple years, after he returned home, to adjust to the rules of the house. There was plenty of times he went to bed angry, but he has thanked me many times for , as he puts it now, being a hardass. I did not expect him to conquer the world by any means, but he was/is expected to obey the rules and be respectful of others' stuff.
My son is now 24, on his own, with a job (he has been fired more than once) but has come to realize it is up to him to put food on his table. He calls me if he desperately needs something....and we help him only if he can tell me what he needs to do to avoid the situation again. So far, knock on wood.....we have only had to pay his phone bill 1 time.
When he lived with me, he tried to run the roost......and yes he did buck the system.
I sat his butt down and 'splained a few things and made sure he understood the other options (homelessness, jail or psych ward).
I cried the first time the po po hauled him off in cuffs (he had a tantrum and was throwing things). The cops sat him in a holding cell for about 3 hours, to get out the cop requested that he write down what he did wrong and how HE was going to fix it.
My situation was different, I didn't have the other "parent" undermining my authority. I believe to get anywhere with the step son you and your wife are going to have to be on the same page. Unfortunately, that may never happen. She needs to understand that an adult "child" will NOT live productively if allowed to make mom and dad so afraid to upset them, they live on eggshells.
Blue, you may have to ask yourself the tough question................Am I better off with, or without them..........and take it from there.
Good Luck with your situation.
Carol