I'd also suggest you at least opt to spend the night in a Value resort. If it weren't for your breakfast reservations, I probably wouldn't; I understand the desire to save a few bucks the night you arrive if it's so late you won't be able to do much besides eat & sleep. Besides, for the money you spend to stay onsite, you could wind up staying at a very nice hotel, maybe with nice onsite dining options or a spa (or you could save cash by Motel 6-ing it).
But you're setting yourself up for a fairly stressful first day because of those breakfast ADRs. As others have said, parking in the Poly really won't save you much, if any, time over parking in the TTC lot (that time of morning, so early before park opening, the lots shouldn't be so crazy). You'd be better off TRYING to park at the Contemporary and walking to to the MK, and there's no guarantee that will work, especially if you've yet to check in to POR and don't have a parking permit. They really do frown on people using resort lots to bypass the theme park lots, though depending on the staff, the season, the day, and other factors you may encounter leniency, but I wouldn't count on it.
On the other hand, you spend a night at Pop, you have the luxury (heh) of already being onsite in the morning. One less trip to make. Bell Services will send your bags to POR if you arrange it beforehand. You can still get a pizza if you want, or anything else in the food court. Take a dip in the pool. You could also take a bus to DTD, if you want. You could drive over to FW, eat at Trails' End and see a movie. (Assuming of course, you don't arrive TOO late) You have some nice low-key options to ease into the WDW state of mind. But most importantly, you can wake up in the morning, shower & bus to MK. When you're done in the MK, some of you can bus to POR and check in (again, Bell Svcs should have sent your bags over, or soon will), and the others can go back to Pop to pick up the car & drive over to POR. By that time, the rest of your family will either be in the room or close to it. In your position, that's what I'd try to do.
