As far as vehicles go, I guess I'd go with a minivan if you must make that drive. That way, all 6 of you will probably get your own seat for that journey. Failing a minivan, maybe an SUV of some sort. Sitting high in a minivan or SUV allows me to drive for longer distances, because it's less tiring to me for some reason.
I know it's against the spirit of your post: but how adamant is your mom about not flying? I cannot imagine driving 2500 miles, one way, just to stay at WDW for 5 days, then drive the same 2500 miles back. Why not just go to
Disneyland? Honestly, as far as attractions go, there's not much in WDW that's not at DL, and DL has some stuff that WDW doesn't have. The weather is almost certain to be better in SoCal in December than just about anywhere else in America, including Florida.
I've often dreamed of driving out west rather than flying, but it would have to be a post-retirement trip probably, with time to stop for a day or two along the way at different spots.