You sound a sensible young mother and a considerate person, and between you, me and the gatepost, your mother-in-law is behaving like all the bad joke mothers-in-law anyone has ever heard of.
Firstly, IMO you are quite right wanting to stay at home at Christmas with so young a baby, and honestly if MIL isn't content with being invited on Christmas morning to open presents she can lump it.
Sit down with your husband and tell him precisely what you have told us here, calmly and nicely. After all she is his mother and you don't want him flying to be defensive because he feels you are getting worked up about nothing.
It might help if he talked to her, and even if you have to, it ought to help that you can tell her that you and your husband are in agreement.
If she turns up just as you have put the little one down for a nap, close the door to his room firmly and insist she is quiet until he has fallen asleep. If she gets hoity-toity about it, ask her to leave and come back later when he is up again.
Going back to work when your son is eight months old should be fine. Ignore anything you are told she is saying behind your back. Ask the people who are telling you these things, not to - you have your hands full coping with MIL without them trying to make things worse.
Have you a FIL who I trust is more reasonable and could perhaps talk to his wife if asked to by your husband?