Right now I'm doing the Pangu's Axe quest, and I'm in the Temple of Heaven but I can't get the axe. It's invisible on the wall but shows up in build mode inside the tomb. It may be because I already have a Pangu's axe in my Sims inventory but I'm not sure. Can I use Master Controller to automatically complete the quest I'm on right now? I canceled the quest and tried to find it in the MC Opportunities list, which showed some of the other adventure quests but none from the Pangu's axe questline.

Not sure if this really helps or not, but MC doesn't provide a way to complete opportunities (in progress or otherwise). It provides a way to begin a fresh opportunity your sims don't already have, if they are qualified to do so, and will not list those being seen as already in progress. It's DebugEnabler that has the ability to force opp completion.

I have DebugEnabler, so how can I use it to complete a current opportunity? Or do I have to cancel the opportunity to complete it in DebugEnabler?

As described here: Opportunities#I-would-like-to-set-an-op ... -I-do-that

Am not sure but I think you might have to cancel an opp in progress to get it to show up. Try it either way, but certainly make an extra save (as) of your game first in case something goes wrong with the DE command.

I have had this issue before as you describe. Try saving the game, exiting back to the desktop, and reloading that save. That worked for me and I was able to get the axe.