Yes, those things all seem mildly annoying, and I know you're claiming you don't let your kids onto your frustration, but I feel like this is what is still really rubbing me the wrong way:
So this year MIL asks if we're planning a party for DS who will be 10, and I simply stated that we didn't do big parties anymore, and just let the birthday child pick out what he or she wants to do for that entire weekend instead.
Do you really think a 14 year old isn't picking up on this? How many other people were at the party? Did you guys have fun after your MIL left, regardless?
Yes, your MIL seems a bit flaky. What she did was borderline rude (as were #2 & #3 on your other examples. #1 seems straight up rude, but I'm wondering what their situation was at the time...was FIL ill? What kind of illness? Were they like this before? Not that I need that info, but it could play into it), but your reaction to it doesn't seem to be "punishing" your MIL (not your job, anyway), but your kids. Plan what you'd like to plan, and just assume she may be flakey. Don't let that ruin your time, and don't let it ruin your kids' time.