Thank you so much for playing and the comment. It means a lot!
It's easy to get blind spots when testing your own game, especially one that's been made in 10 days. So your feedback has been very helpful! I'll use some of the points you mentioned to improve it in a post-jam version. :)
SPOILERS BELOW
I agree, the game doesn't make it clear enough that brimstone is sulfur. The only clue is when you're speaking to Elijah the engineer, he says he smelled brimstone and you, in turn, ask him when he smelled the sulfur. As for the letter "L", it's meant to be filled in last and since it was the only letter left after you've filled all the other letters, I didn't add it to clue set 3 (Maybe I should have though.). Your point about undoing is very good, I'll make it clearer when I update the game.