There is no answers to these puzzling why's. I don't even think Kay knows why. Her best answer was "It's free not to have to join the rat race and work." Any further insight she doesn't seem to have. WE can guess but it's just guessed.
Obviously not all kids are out flourishing. All of us are in your situation! I don't know about you, but I tend not to talk about Kay. I don't feel the need to be judged and we did our very best. So people hear about Amy and Rick and not Kay. If they ask how she is I just say "She's in Arizona now doing better." I won't say more.
Our own guesses, which could be wrong, are that Kay's adoption status and her inability to achieve at our family's level, mental illness and drug abuse all factor in. Are we sure? Nope. Agree that there are often no answers. It just is.
Your son probably would feel relieved if his warrant were not hanging over his head. He is young and could see the light.
I have a good book that helped me, Radical Acceptance by Tara Brach. Great concept!
Hugs and prayers.