3. Would you still support a mega dungeon if it diverted resources from the main plot/game? My guess is no, but I had to ask.
HELL NO! A game can't be everything to everyone. If you want to make a story driven game, make one, if you want to make a dungeon crawler, make another one. Never mind the fact that these concepts tend to appeal to different people, if you try to do both simultaneously, likely neither one will come out terribly well.
What kind of sense would it make to take a well-paced, narrative driven game and just crowbar a major timesink in the form a mega dungeon right in the middle of it? It's going to kill the pacing, feel out of place, and likely will be too long and boring for people who play for the story and not nearly long enough for people who like dungeon crawlers.
It's a pretty bad idea to begin with, but do all this at the expense of the main plot on top of that? No, no, NO! Might be the worst idea I've seen on this forum thus far.