Minetest's screwdriver is super unintuitive and I'm surprised you went for a game with it as one of the core mechanics.
Enhance the contrast between detector on and detector off. You should be able to tell the difference at a glance.
You shouldn't need to click the emitters to turn them on. They should always be on.
The level change is JARRING and leaves the player no time to understand the solution they just made. I completed a few levels without knowing how.
No level select
Level 9 has 7 emitters, 7 detectors, and 7 mirrors, but since one of the emitters isn't at a right angle to any detectors, it should require 8 mirrors (6 right-angles and 1 u-turn). Thus I got stuck on level 9.
I agree with everything you said, except the thumbs up. :D
I have the same criticisms of my own game.
I would have given the game a thumbs down.
I am aware of all of the issues, I just RUSHED the development in the FINAL DAY because I procastinated. :-( So obviously there's incompleteness.
Please stay tuned for a future version, because I do like the game idea (just not the mess of a game jam version, lol)