Hi Mel and welcome to SR.
It's interesting that you say your AW's excuse for not going to re-hab is because she can't bear to be parted from your son. What is her drinking doing to him? And, you worry about destroying your AW's world in the event of a divorce because your son is, as you put it, "her life". Well that isn't quite true....alcohol is your AW's life and everything else comes second to that.
I feel for the children in these situations. I worry about the examples that are being set and the quality of care they are receiving and more so in your situation, Mel, as your son has special needs. Once he begins formal schooling, the school is going to be wanting to work WITH both parents.
Mel, don't beat yourself up. Read here, ask questions, go to Alanon and make informed decisions as to what is best for you and your son. You can control you, your son really only has you and you have no control over your W's disease. I'd be careful about giving ultimatums unless you are seriously going to follow through...otherwise the A knows you weren't really serious and the cycle continues.
ARL