Alright, I got through that puzzle as well. I must have spent close to ten hours in this mod already, congratulations on capturing the challenge of the original DM series. Indeed, in the old days before the Internet, the solutions could only rise from trying out even the non-obvious in frustration.
Your mod is a true gem.
However, hiding a key in a pit was still bad taste. The dungeon really doesn't want to train completionist players to jump into every pit to "discover its secrets"! This is akin to forcing them to bump into every wall in order to discover secret (invisible) passages.
Some other things to consider - a non-linear dungeon keeps players on their toes, but in my current order of completion, I have to travel through bottom floor to reach different unsolved puzzles in the dungeon, and this sometimes includes jumping to a pit as well. This does not increase the entertainment value, it is simply a chore.
It would be great if you could indicate solvable and unsolvable puzzles better to the player so that he would not have to spent hours trying to do the impossible. So far, I have given up on SW puzzle on floor 4 - I got the blue gem all right, but I think the small locked room with two crabs seem to be unreachable for now. Likewise, the center of floor 3 has this corridor with a trigger plate that closes a door at the end of the corridor, and barring new discoveries, I do not think the player can get to the open door past the plate. In both cases, it may of course be that I have simply missed the "obvious" solutions.
The puzzles are devious and sometimes diabolical, which is a good thing, but in order to prevent stubborn players from getting stuck "unfairly", you might also consider the "three keys out of four" approach that has successfully been deployed before. In other words, solving three difficult puzzles out of four should be enough to get to proceed, and solving all four should the player some extra treasure.
Thanks again for the dungeon so far, I am sure it will keep entertaining me right to the end.
Edit: Found out the truth about the "crab corner" later, and sure enough getting more doors open makes movement easier. I still dislike having to save-load and jump and/or throw stuff into every pit to make sure the solution does not lie there.