Gidday Todda and thanks for your emotional and well thought out post!
I sounds like you are an extremely competent, compassionate and experienced step parent - working in child care, having tried counselling, contracts, boundaries etc. So I am not really in a position to offer much advice at all, apart from sharing my own story.
I am on my second marriage and my new wife inherited my two teenage daughters. It was very, very difficult for a long time despite her unfailing efforts to be kind and helpful to them. They made it very hard - a story we hear is not uncommon at all.
What eventually happened to change things was:
1. I genuinely loved her.
2. She was NOT a replacement for their mum.
3. The kindness and interest she had in their wellbeing was genuine, not fake.
4. She never, never got in the way of the relationship they had with their mum.
5. My daughters grew up.
Not it sounds like your stepson in 19 yrs old? Interestingly, my daughters were at their most difficult when 15-20 yrs of age, then changed quite quickly after that. So these days our family is very loving and functional. What a change!
So hang in there Todda - keep talking with your step son, show respect and treat him like a human, and hope that he too will grow out of his funky mindspace.
Just a thought - have you tried asking him how you can help him with his life at all, is there anything you can do, is there anything at all he wants to talk about with you?
I am very happy to chat further with you if that might help.
All the very best - be strong! The Bro.