Damn, that really sucks. Well, the gloves are off. And its very unfair to you who would like to do this as amicably and as considerately to li'l lookOut as possible. Protect your son and yourself as you would against anyone who would bring stress and disruption into your lives. I don't see why she should get a "free pass" to wreak havoc at your expense.
I know you don't feel this way and I understand but if it were me, I'd take her ass to the mat.
But, in spite of the bumpy ride ahead - it won't be as bad as it seems right now. Check in and post updates. I've helped a lot of friends through similar situations and this one is shaping up the same way. I'll fast forward to the end: you and lookOut Jr. will bond together and both of you will come out of this better, stronger and happier.
And if you ever find yourself in a spot and you just don't know what to do, do what I do - ask what John Belushi would have done and do that.