You should get a van. The hotel is usually overpriced by a huge amount. Given your situation (need a van not a regular car) and two different hotels, I'd not try to deal with negotiating "on the street." Try this company for an alternative to the hotel--they have a reputation for reliability. I don't think their full day (8 hour) rate for a van is bad--I was going to estimate a "going rate" for a van at about RMB 1000 and this is right there in that ballpark. Divided 5 ways...I'd do it. You should be able to contract the arrangements in advance and with an English speaker.

Now to your arrangements. Summer Palace, absolutely over the Tombs.

Doing both Mutianyu Great Wall and the Summer Palace successfully in the same day ABSOLUTELY DEPENDS on being able to leave the hotel by about 7:30 in the morning, and for central hotels like yours, 7:00 is better. And I mean van loaded and pulling away from the hotel door--or in your case, the 2nd hotel door. If your group is not the kind that can do this, then do not attempt both these sites on the same day.

There isn't anything wrong with the hotels or the neighborhoods per se, it's just that traffic patterns around each, and particularly this Marriott, are NOT conducive to doing a double pickup and getting on the road early. Basically, you'll get mired in some of the worst traffic in the city. If you cannot change hotels, then I suggest that the group at the Marriott finish breakfast and get in a taxi at 6:45 am, then taxi over to the Wangfujing Hilton and all of you go from there. The two hotels aren't far and the fare won't be expensive, but you want to bridge the gap before traffic starts to go to gridlock. Getting everybody corralled at one spot would also be easier to deal with explaining van arrangement to the provider.