I sympathesize with you Cheyenne. MILs are a handful to say the least. The way I see it, there are several issues to address -
1. Tomb-sweeping - short term solution would be to think creatively and come up with something to prevent your baby from having to go (doctor's appt/feign illness). Longer term solution would be to get your husband's consensus that baby shouldn't go for such trips. Oncw husband agrees, then it will be easier to speak to mother in law (relying on the fact that his sibling also doesn't allow their kid to go);
2. Issues with your mother in law in general - she will not change neither is she possibly going to admit her errors. To minimise conflict, you must explore living seperately from her. This could mean moving with your husband and kids back to your parent' place or even renting. The distance alone will give your some peace of mind and it will be easier to tolerate her during the few occasions when you meet. Again speak to your husband and get his agreement on this;
3. Issues with your husband - I agree with someone else who has posted here- your husband does nto sound weak but rather confused and caught in the middle. When you speak to him give him alternatives he can consider that he would be more inclined to accept. For instance - even after your confinement, you could consider staying with your folks for part of the week (weekdays?) and with your mother in law during weekend? or get your husband to have his mother involved in some activities during the times when you are at home (shopping, mj, holidaying - anything to reduce her presence at home or get mother in law to stay with another child for a period).
It is clear that you have been tolerating your mother in law for a while and you feel that your husband is not supportive enough. Sometimes a different approach to the issues may give you a better result and often the result that you are looking for. Hang in there and think about how you can connect with your husbnd better.
Hugs to you.