Sorry for your loss. It will bring up all sorts of emotions and reflections. Also, unfortunately a lot of admin too. Especially a she wasn’t very organised - what a headache!
Well, you certainly sound like you have a wise, insightful, head on your shoulders, but unpacking things with a therapist is hardly ever unhelpful.
It can be so hard to relate to and understand people like your mum, and maybe dad too, because you are not like her, same for MiL. This can be what causes us to keep being hurt and disappointed because we expect more than they’re capable of. It sounds like you turned a corner though when you decided to match her energy. Well done. It’ll get easier. Accepting that you are not obligated is a bit help.
Minimal contact can be very liberating, as can no contact, however I appreciate the things that are holding you back on that one.
Maybe some conversations with MiL about your experiences may be helpful? However, it sounds like if mother decided to try to manipulate MiL, she wouldn’t see it, or be able to assert against it?
Maybe just keep up the minimal, energy matching, arrangement you’re already doing. And I am sure there can be a few last minute reasons you may have to cancel too, or why the kids can’t come, to drag things out even further, if need be. It may end up being just a few awkward hours every other year.
It may be the best way to not cause a battle with her Ego, but accepting this and why, also that it’s not your fault, we don’t get to choose our parents, can be really helpful. The more we see it for what it really is, the less hurtful it is.
You are not like her, or him, by the sounds of it. That is the main thing.
All the best!