Ah, the "Missing quest." I think the only purpose for that is to activate the teleporter to get to the other islands. If you've tried to use that teleporter prior to going to talk to the homeowner, you'll find that it doesn't work.
I guess the "Reward" is what little experience you get from slaying the bandits, as well as the treasure that you find along the way. And, of course, the unlimited use of that teleporter.
As to the torch in the sewer...that is a difficult call. Keep in mind that in some other dungeons, gold is hidden behind a couple of torches...and in yet another dungeon, there's a "Flickering torch," which opens a secret panel that reveals a button that needs to be pushed to open a door farther along. So, early on, I made it a habit to at least hover the cursor over torches (And everything else,) to see what the description says.
Of course, I usually clear out the thieves first anyway, so by the time I get around to that torch, nothing else happens. I suspect that perhaps they'd wanted to have that tied to the walls where monsters are stocked in the sewer to release those monsters, but -- for whatever reason, decided not to. Or, perhaps they'd wanted to hide the sewer key there instead of elsewhere (And, if you find said key in MM7, it has a nice nod to the missing key!)
Plenty of hypotheses, but I wonder if syntax, or memory issues, or some combination thereof, prevented them from going any farther with it.
Inquiring minds definitely want to know!