I agree with Zahra- I wouldn't expect any compensation from her, and I wouldn't want to get tangled up in court with her, but I would definitely want her to know what happened- she probably remembers it very differently than it actually happened since she was so drunk. It's a pretty tricky situation, but you keep mentioning how socially prominent and wealthy she is, and I definitely wouldn't let something like that get in the way of me confronting her about it. That's probably why she acts like that- no one ever holds her accountable for her behavior! I understand the benefit to keeping civil with the in-laws, but I think there's a way you can navigate this so that you can both inform her of her shameful behavior and keep the peace. In the letter, I wouldn't act angry or indignant, but I would act like I very much pitied her, like this:
I noticed at my daughter's wedding last week that you were not your usual self, and I hope you are feeling better now. I don't want to cause any friction between us or our families, but I did want to discreetly let you know that many people at the wedding were very concerned about you when you started to serve the cake. I know you would never intentionally ruin such a lovely piece of art or knowingly prevent other people from enjoying it, but wedding cakes are difficult to cut without making a mess out of them, which is why we had our baker specifically instruct our friends X and Y in how to properly cut and serve the cake. I'm sure you're not aware that the cake was rendered inedible, even the lovely anniversary tier, after the change in servers. I've enclosed a few pictures of the cake after it was ruined, and I'm sure you can see why so many people were concerned for your state of mind. I wish you the best, and again, hope that you have made a full and speedy recovery. Please let me know if there is anything I can do for you.