We did this last year. I put down the deposit with my Disney VISA, then when it came payoff time, DH used a joint cc he has with his Mom, and she paid for the balance of the trip by him using that cc. You only have to make the deposit using the Disney Visa.
I'm not sure about the multiple rooms--you could call and ask if nobody here knows. I don't think it would be a problem, but I've never tried it. I would think, so long as the deposit is made with the Disney Visa, you're good.
A couple thoughts, though, on planning for such a group. Are you confident that they'll pay their bills in time? If they don't, you'd be out the $200 deposit per room. I only ask this because there have been many, many threads on these boards about people planning group vacations, then having someone flake out. You could ask them for the deposit money upfront--then they're the ones who are out the $$ if they don't pay.
Another thought--if the family is good about paying, you could put the entire cost of the 4 rooms on your cc, pay it back with the money from your family, then use the rewards points on your trip. I would only do this if I trust them--my MIL is always scrupulous about paying us the few times she's gone that route, but I wouldn't put out a dime for my BIL (hosed too many times).
A few general thoughts on planning for a large group--we've done family travel (MIL, BIL and his family, and us). We found it works best if you're all in the same park, same day, but not try to move in lock-step. We would meet up for an agreed-upon dinner reservation, which would give us all a chance to catch up on the day. And of course, we'd see each other in the parks, sometimes break into small groups to do a few things, etc. Doing this allows the "uber-planner rope-drop" types to do their thing, and the "sleep in, we'll get there eventually" types to enjoy their vacation.
