I suppose you simply jumped down the pit to get to the other side of the gate. That's what everybody does. Instead of that, you could also just throw the green gem into the pit. It will land on the tile where the chest that contained usually was, and will open the gate, so you don't have to jump into the pit yourself.
Yes, it is probably the only badly-designed puzzle in the whole dungeon. It would have been nice if there were no pits at all and you had to throw the gem through the bars, so it would land on a pressure pad... Hey, that might be an idea for my custom dungeon...
Regarding the room upstairs where you drop down the pit. I remember from playing on the Atari years ago, that its possible to operate the wall switch and run into the opening space quickly and be trapped forever ! Dont know if this still is the case. must try it sometime.
Is there a way of getting the Diamond Edge without taking any hits from the poison darts ?
I picked up the Diamond Sword and took the poison cloud and waited for it to clear. I then throw the Diamond Sword down the corridor. Walk until I get it again. And repeat this until the sword is all way out the door. I got no poison darts or any other poison clouds doing it this way.