In term of the crews, AFAICT, based on AFA, there is no Mainland China based crew. There are only HKG based crews. But I could be wrong about this.

Because HKG based crews are under the same CBA, so if UA can't accommodate the scheduling, they will be paid based on the minimum guarantee flight hours. Also - HKG based crews all have D Visa if not being American, allowing them to work for UA in the U.S. In fact, HKG based crews have done domestic duties before.