My prayers are with you, DC. I'm all too familiar with the dementia experience. My father suffered from it for years and fought all attempts to get him to enter a facility or even get the most basic treatment. By the end I was just hanging on for dear life, praying every night that I could make it through one more day. My point is I did make it through -- with God's help, I believe -- and things did eventually get better. I believe you can even find a solution to the problems with your stepson if you just hang in there. I don't have kids but I certainly have had a few friends who have snapped. It can be very disturbing and in my experience even threatening. But if you just stay centered and try to keep your feet on the ground you'll eventually find a way out.
"Got any nails?"
"Got any flies?"