We had a very similar experience as you. We laid out there the expectations before we left and by the end I was done. I felt like a travel agent and the bad guy trying to get everyone to the fastpasses and dining reservations. We set up the dining reservations and fastpasses to accommodate the other side of the family. Like I said in my other post, we were even changing fastpasses for when we thought they would be in the park. It is hard to mix different types of vacationers in this type of trip. I probably won't do it again.There are so many things to think about..... Parents with a 15 year old are going to have different plans and must do's that the parents of the 1 year old. The 1 year old will need naps where the older kids may not. Because of this and other reasons I would say the more rooms/space you can have the better.
When it comes to meal plans they may be ok if everybody is on a separate reservation. 12 people on 1 reservation with a mixture of adult and child credits makes me really nervous. The other thing to think about is tips and extras. If parents "want to pay for everything" then gift cards may be the way to go so that all dining expenses can be charged to the gift cards. Or have everything charged to the room with the parents card as method of payment.
When it comes to MDE make sure each family has their own account. Putting them all on one can be a disaster if they need to be split up or adjusted in the future. I would offer to set them up for everybody and then email them their username and password. You can link them to your account so you can include them on changes if needed.
Before you leave make sure everybody is clear on expectations. We did a family of 11 trip a couple of years back and by the end we said we would never go with family again. As we were the Disney veterans there was a level of expectation that we would "hold their hand" from checking in at the airport through the end of their trip (we stayed 3 extra days). We had connecting rooms and by the end of 4th or 5th day we would quietly get ready and just go. We are all about helping family have a great time but in the end it was our vacation too.