Great feedback! Aside from stuff that we couldn't do due to time restraints (or, more exactly, stuff we didn't think we'd have time to fully implement), here's my comments on your bullet points:
-There's a sound effect when you lose a key, though that might've not been loud enough... The sound effect system had to be rushed together, so we didn't have much time to check if everything was at a nice volume :(
-The order at which the keys disappear is random. In hindsight, this was probably a bad idea...
-One of our dev team members actually made a similar complaint to the movement system. While I do agree that it feels strange, I'll take the blame for this one - I did it this way because it was easier to program and gave the game a more retro feeling.
-We didn't add a "more keys" powerup for two reasons: 1. It could've stretched the game for too long - We wanted hits to be permanent, and the game to be short. 2. It could've been buggy to implement, given the time restraint.
-You're completely right with your "not engaging with the enemy" comment. This actually ties well with the "new enemy types" bullet point, too - my idea was to have the threat of increasingly more dangerous waves pushing the player towards getting as many stars as possible before getting swarmed. It would've been nice to have more varied types of ships instead of just the same enemy x100, but again, we didn't think we'd have time to implement it. In hindsight, I could've probably made at least one more type of enemy, but I didn't want to risk working on something that wouldn't pan out in the end.
-Finally, the buggy buttons: I'm really sorry. I spent a good amount of time trying to fix them, but the problem seemed to be tied with Godot's collision exit detector. After pouring a lot of time into it, I eventually gave up.
We're still discussing on whether we'll keep working on this game. If we do, I'll be sure to take your feedback in consideration! Again, thank you so much!