Little wonder xmas is a tense time of year for you. I've always however, never thought much of anniversaries, my dads passing for example comes and goes and I dont connect the date. The reason is that dates and time of the year is a man made thing, the same date for example means the earth is at approximately at the same point to the sun as it was when he passed on. So some people get caught up on the anniversary of whatever and get upset. Another example is a friend of mine that, at 3pm on her mothers anniversary, the time she passed away, she would grieve for a few minutes. What she didnt realise was daylight saving meant she was one hour out of her time frame. So effectively she wasnt coordinating the right time. I dont know if that makes sense or not to you. If you had that concept then xmas would be less effected by your past events.
In laws especially parents inlaws can be a huge issue in a relationship. I personally have never believed a relationship should be held together for the sake of children because they are indirectly effected by our marriage tensions. Children are resilient, they can adapt surprisingly well upon separation as long as their isnt bitterness and ongoing disputes, which, with your really good attitude in that regard will be fine.
If you feel there is some hope for your marriage I would consider family counseling. That will at least give your partner an opportunity to open up about his feelings even though much of it, about his mum, you wont like. That opportunity will give you either lots of hope to repair it or give you more closer to move on and separate.
Certainly boundaries need to be put in place with the input of your MIL and changes to how your husband is committed to you in terms of getting the balance right. Some parents make their children their lives rather than have activities away from them, even when their children are adults. A counselor will be able to identify if your MIL has jealousy or insecurity issues or if your husband has issue also, for example communicating to you better with his plan to renovate his mothers place and the balance of satisfying you both as a couple.
I hope you feel better soon.