J.S.
Hi K. - It sounds like you're a smart, compassionate person who's in a tough spot. I have a hard time with similar issues in my life. How do you be generous and loving because you understand a person's situation and feelings, yet not enable or perpetuate their bad behavior?
I would say that you should do your best to always be doing right by your MIL because as your husband's wife she deserves respect. If someone is in the wrong, it should be her, not you, because you can only control your own behavior. Do what you can to be reasonably supportive with regard to her financial state, and to give her time with her grandchild as you would if you had a happy, healthy relationship. But don't let her turn your relationship into a contract! If she loves her son and her grandson, then she will naturally want to be with you all. But don't let her have the upper hand and bargain for your time, *especially* if she requires payment for it!!
Do anything you can to find alternate care for your son so that you never have to rely on your MIL. Her time with him should always be voluntary. If she offers to babysit, then kindly accept it without feeling any need to pay her. It sounds like she has quite a feeling of entitlement - she wants to be cared for without having to work, she wants to be near her son and grandson on her terms, and if you move away then she turns it into you trying to be nasty to her. Sorry to say that in my life I've seen this behavior a lot from people. (My husband is a pastor, so we've been in a lot of counseling situations!)
Take one day at a time, knowing that there is a finite number of days until you are moved away, and that each day completed will not ever come back again. Do your best so that at the end of each of these days you feel good about your behavior and that you made a good example for your little fellow to absorb into his developing personality. I know it's hard to be the one in the sacrificing spot, but sometimes that's what we're called to do. Pray for help!!!
(Hooray for you being so respectful of your husband - it's your half of what makes your marriage work!!)