I finally completed the game, restarted a little less than 10 times in total I think.
I guess the yellow key is meant to open the already opened door to the end, not that it changes anything, you need to grind all the enemies to deal with the queen anyway. It was super tense, I ended up one hit away from dying with no potion and the queen did not attack the last time. I did not open a chest behind a grid, I assume you can fall down into the room somehow.
The game is fantastic, it looks and sounds great and is super atmospheric, and I like the push-your-luck combat and potion management, they are perfectly adapted to a game of this length without saving option.
Congratulations!