This may sound weird to you, but 19 is still pretty young for some people. I wasn’t self-sufficient until I was 26 or so, and I had a normal childhood and went to an elite college. (I was also clinically depressed and kind of a momma’s boy.) Add to your step-son’s case that he was abused, etc. and all that sounds about right to me. He’s missing a lot of lessons and examples of manhood and adulthood. Naturally, being insulated from the the “real world” as he is now and whether it’s from school or work isn’t helping. I would guess that he’s not following through, because he doesn’t have a tangible sense of how the process works and because he is capable incapable of doing it without significant hand-holding. He probably would benefit from mentoring from you or someone else (but not his mom.)
In light of your having reached your breaking point, perhaps you would do well to have a family meeting of sorts and show them the facts and hard edges of your family’s budget, and who’s contributing what and then having a discussion about what is fair give and take for every member of the household, and even include some aspirations for the future (e.g. saving for a vacation). Here you would get to show your share of give and take and what your personal boundaries are. When they protest with excuses, etc. you can say that’s all well and good, but I feel I am deserving of x, y & z standards being met, and it’s not fair to me to have to put up with less (i.e. it’s insulting). So, yes, I love you guys, etc, but the bottom line is that I can’t martyr myself for your welfare if you’re not trying and not respecting my effort by putting out some of your own (because if I do that, there won’t be a me left). And then you have to back that up with consequences.
I’m telling you, though, you’re not going to get anywhere without your wife on board, so you might as well focus your attention on her. She needs to get past the guilt, and the trauma of being battered or whatever and realize that the best thing she can do for her kids is help them heal by acclimating the whole family to an emotionally healthier lifestyle. She needs to get out of the tug of war match with her ex and learn to react differently (i.e. as a better parent) to her kids’ manipulations.
I realize this is coming across as super touchy-feely, but I think it’s either a lot of up front and sustained patience, hand-holding, and encouraging of baby steps on your part or an inevitable walk out the door. Obviously, nothing you’ve done so far has worked, so if you believe any of the above, I would either strap on a helmet for the long haul or come to the realization that this isn’t how you want to spend the next couple of years of your life and move on. (Or just keep doing what you’re doing and delay your inevitable burn out and departure.) We are all broken people in different ways, but not all of us have the capacity to fix every problem. What you have is a big bundle of problem and three people to “fix” if life is going to be to your satisfaction.
I will say, though, that if you take the time to lay a good foundation, your efforts will probably start to pay off quicker down the line, because as each member of the family gains a little more momentum it will make future changes easier.