"Do you think resetting the game can help?"
Not sure, but worth a try.
"Also, is it intended, that the enemies go down in two hits after the first boss "battle", instead of in three they took initially?"
It's like you're getting stronger =]
Alright, I've finished the game, and now I get how it adheres to the theme (no spoilers). :-) I also think I've figured out what caused my key to disappear: after defeating the boss, another player character spawns on the opposite side of the map for some reason, and it also responds to the controls, so when I move into the dark room, it can easily fall into the pit and lose "my" key. I worked around it by walking over him, so both "my" PC and the clone are in the same cell at all times, but the game eventually hung up when I tried to enter the dark room again. I was able to beat the game after the restart.
Also, not sure if it's a bug or a feature: the attack option during the final boss fight reads only "Let", and in the final log, I can only see what appears to be parts of the text. Is it supposed to be like this, or is it just the display elements eating up large fonts?
"another player character spawns on the opposite side of the map for some reason [...] and in the final log, I can only see what appears to be parts of the text [...]"
Indeed. This is part of a last minute error that could not be fixed before the deadline.
A properly functioning post-jam version will be published as soon as the system unlocks the submission of new builds (after the voting period ends).