I had similar problems with a different quest line (one that starts in France and requires you to travel to Egypt). I had to send the sim back to France to get the quest again and then travel to Egypt (about a dozen times before it finally worked). Trying to spawn the opportunity via MC never worked for me. I don't recall if I also had problems with the Dragon Cave quests.
I have completed the Dragon Cave quest line, and did have the amulet. In addition to the amulet and training martial arts, there's another intermediate quest to actually win the tournament. I think you've got at least 3 quests after coming back with the key before your sim battles Don Hua.
So, drawing on different experiences I've had with WA: I would try going to France and trying to pick up the adventure on that adventure board, then traveling to China. If you are using Nraas Traveler you can travel directly from one WA world to another. I don't know if that's possible without Traveler.
I did not use the key to the inner sanctum part before I was directed to do so in the adventure line. I did explore part of the cave earlier though.
Good luck.