Planning a multigenerational trip for 12 to WDW probably feels like planning the D-Day invasion of Normandy.
We went to
Disneyland this past spring with my MIL (who was in a
scooter), so it was just a total of 5 of us and I found THAT to be overwhelming. Why? Mostly because while I knew my immediate family's preferences and even though we had discussions w/MIL beforehand about stuff she wanted to do, places she wanted to eat, etc., I KNEW that last minute stuff would come up and sometimes changing plans on the fly can be hard.
So case in point:
We were there for 3 days with her. In the evening of day 2, she said that she wanted to do a little bit of shopping. Great, my DH took her to go to that. Oh no, we found out on the drive home that what she REALLY wanted was for ALL of us to spend a few HOURS strolling through multiple stores with her while she window shopped. AAAHHH! She never spoke up about what she REALLY wanted. Never said that she wanted to go into the New Orleans Square stores...she said in the moment that the Main Street shops were fine.
I also found it to be really stressful because it felt like I constantly had to check in w/her and make sure that she was ok, did she need a bathroom, did she need a rest, etc. because she won't speak up for what she needs. AND Grandma is hard of hearing, so we all had to shout at her the whole time. And she drove a scooter...and promptly ran into so many people that I lost count. One time, she even accidentally stole somebody else's scooter. It was exhausting.
The best parts of that trip were when I finally threw caution to the wind and got the attitude of "Forget it! It doesn't matter what I do...someone is bound to get irritated, annoyed, etc. There's not much I can do about that. I've done the best I can and whether or not the person chooses to have a good time is NOT my responsibility."
Splitting up was definitely a big plus. Parking tired grandma on a park bench while we went on a thrill ride was also a good move. We did that a few times. We also went in with the attitude that it was ok to change our plans on the fly so if somebody didn't want to go on a particular ride or whatever, that was fine, but that wouldn't mean that the rest of the group had to sit it out, too.
I think that you should consider having every person in your group pick just ONE "must do" for the entire trip. Then try to get their "must do" accomplished in the first part of the trip. And everybody needs to have the attitude of "everything else in addition to that is icing on the cake."
Don't require everyone to rope drop. It probably won't happen.
Don't require everyone to spend every moment of the day together.
EXPECT that you all will get annoyed with each other. PLAN for down time away from each other EVERY DAY.