Hi! I agree with everyone, just focus on the divorce as business and let the lawyers do the work. When I had to make a decision my sole thought was whether it would benefit my children, and I've never regretted that.
I agree with your uneasiness about letting your MIL use your home as base when your STBX has his own home. From what you describe, using her own key to let herself in, she has boundary issues, and hasn't communicated with you in some time. Your STBX, and his M, need to start experiencing the consequences of his actions. After divorce, they will not be able to use your home as base when it's his time to watch your child. He needs to set up his home now and start parenting; if he hasn't already done so now is a good time.
