Firstly, I would ask @MNHQto move your thread over to the Parents of Adult Children section as there are other MNers dealing with abusive and addictive adult DC in there.
Then, please stop feeling guilty about leaving his DF. It sounds as though you were in an abusive relationship and made the best choice you could at the time.
I tbink the Divorce rate is currently around one in two. I’m not sure ehat the figures are for parents who have DC then split but it’s going to be high.
So plenty of adults come from homes where there is one main parent and they don’t turn into addicted, abusive adults.
He’s using you splitting with his DF as an excuse and if it wasn’t that, he’d probably find a different excuse to drink.
So definitely don’t condone his behaviour. He needs to sober up and quickly before he loses his DP, his DTs and potentially his home.
I would speak to Al Anon for some support for you and suggest to his DP that she does the same.
And I’m sorry, seeing your DS parent his own DC and manage his adult relationships so appallingly must be so hard 