But the main thing that stands out to me from your situation, is this:
He is still heavily involved with her.
I don't mean he is shagging her, or living with her or anything like that. I mean he is arguing with her.
this is massive. It is a huge attachment. It is still a relationship - albeit a perverse and dysfunctional one.
He has never walked away from it. He is allowing it, he is engaging with her, and he is not protecting you.
Basically, you are an accessory to their ongoing relationship - they are so deeply entangled that I doubt they will ever be free of each other. They could be - if theywanted to it is easy enough to disengage.
But they choose to be entangled. I would be stepping away till he is genuinely on his own and has very little to do with her - no arguing, no dissing, no upset.
All the time these things are happening, you are very much on the peripheries - you're not very relevant. Its not about you.
Does that make any sense? He is still putting her as his priority in the mental and emotional energy she takes up, because to him, she is more important than you are.
And should you walk away from him I have no doubt he would go straight back to her in the other way as well. He's trying to maintain a three legged relationship, atm, and both of you are necessary...if you get out of it, he will need to go right back to her in every way. He's using you both.