Just hosted my family in Beijing and brought them up to Mutianyu for a day trip.

One could always ask your hotel to negotiate with a taxi for a day rate. They'll most likely be more than happy to oblige. 500RMB is more than fair and should include all tolls and parking.

With my limited Chinese I was able to accomplish this no sweat, but having your hotel help might be a better option.

There's really no need for an English driver, once you get there, just make note of where you're parked, and follow everyone else. Take your drivers mobile number too just in case I suppose, but he'll be waiting, since he wants to get paid at the end of the trip.