If you get a loaner pop-up I would strongly advise taking a weekend trip with it in the fall to set it up, take it down, and get used to towing it (with the vehicle you plan to use for the Disney trip). Having a tent helps off-load some of the capacity issues related to your crew.
I would NOT advise towing the loaner pup for the first time when you leave for WDW in Dec. But that's just me. And as others have said, when towing you have to know your weight limits. A minivan is NOT a great tow vehicle in some cases for a pup (trust me I know). Most minivans will tow a max of 3500 pounds (and that's with the tow pkg) so you need to know the weights fully packed for the pup you have. If it has a slide out it's close to 3000# on a 12-foot box. If no slide, 2500# fully loaded, packed, and prepped. Then you add the extra passengers to the car, suitcases, etc. and it's not hard to buy a new set of brakes.
Even if you rent the pup or pull one from home this will work. In a minivan you'll take about a 5 mpg hit on fuel and it is very sensitive to speed (60 mph gives much better mileage - maybe 2 mpg - than 70 would if you pull that fast).
1. Make your ressie now for a site. You can always cancel it up until about a week or so before arrival and they need only 1 night fee for deposit.
2. That gives you time to decide rent-or-loaner.
3. In addition to your ideas we also save money by packing a picnic lunch into the parks. Disney will let you as long as there's no glass (except baby food if needed) and no alchohol. Soft sided coolers work best. We rent a locker first thing going into each park and stash the lunch so we don't have to carry it around all day.
The crowds will be biggest through New Years weekend so you might choose to go to the parks after that for smaller crowds and enjoy the Fort decorations and activites. Lots of people decorate for Christmas and the Fort has kids activities 2-3 per day which are listed on the sheet with the movie schedule when you check in. I'll see if I can get mine from last week scanned and post it.
Weather into early Jan can be dicy but if you're from Michigan then it will be nothing new to you.
Good luck to you
Bama ED