We've done a few trips with the extended family and it's never been a problem. I think setting expectations beforehand is a good idea. I did/do ALL the trip planning: airfare, resorts, dining, park days/times, rides, etc. I verify with everyone prior that this is what I will be doing, but if they're more comfortable booking on their own, then that's great (no one does). I also remind everyone daily of the next day's activities. If my dad doesn't want to leave that early he will meet us later; if my mom gets tired and wants to go back early, she will. The one thing I do ask is that if you planned to eat a table service with us, then show up
I remind my mom and dad that it gets hot, and there's a lot of walking. They've never been uncomfortable with saying they had to leave park early or sleep in a bit.
We've had times where my son only wanted to ride with his cousin, or poor grandpa was always stuck with me
If that's the case we all do a "partner switch" on one ride at end of day (ride with someone you haven't yet). I like to get pictures of everyone with their ride partners, too, and everyone seems to enjoy that (especially if it's one with 3D glasses).
With grandparents buying things we've never had a problem. My kids are a little older, so not sure if this will work on yours, but from the time my kids were little I talked to them before we left (and reminded them in our room at night), that they don't
NEED to get something every time a grandparent offers. Think about it and decide if that's what they really want. OR, just tell your parents/in-laws to wait until the very last day and then go ahead and buy (chances are if it's in one park, it's in every park). Generally my dad will give the older kids money to buy what they want and he'll give me the money for the little ones to pick out what they want.
I'm not sure what you mean about your mom being jealous, but I can tell you I've had a "complainer" on our vacations before and it can really, really bum me out. I don't like that feeling. If you know your mom will get like that can you talk to her before you leave? I tell everyone WDW is the happiest place on earth and I don't want to hear complaining or negativity while we're there. 99.9% of the time it works.