Mutianyu is the best choice of the three for December. Jinshanling is too far for the short days. Badaling not as impressive and will have more visitors. You may safely skip the Ming Tombs, secure in the knowledge that you haven't missed much.

The "gauntlet" at Mutianyu isn't bad or particularly aggressive. It's just the path that leads up to the walking path, cable car, and chairlift accesses. There are little souvenir shops and some eateries lining the path, some with touts that may call out to you. But they aren't grabby or anything, you just walk past, no big deal. The biggest stalkers tend to be the ladies selling souvenir booklets and post cards, but just shake your head and ignore them.

Since it is very cold and windy in December (bundle up including head and hands), both tourist crowds and stalkers are in shorter supply than in tourist season.