Hey thank you for playing, and for your detailed feedback! Good point about the hud, that one's on me haha. As for the lemon, the mermaid's fish half is a lemon shark! Look them up, they're neat :) One of the dialogue lines while swimming down the tunnel mentions it. We figured that it would be enough to qualify as a "hint" of lemon, haha.
Regarding the boss, if you died it should have respawned you before the fight, but I think there's an issue with that checkpoint specifically. I'll have to investigate what happens there. Not sure what you mean by some enemies not dying properly, but they do also do some weird things sometimes where they chase you way too fast, so it might be a bug i haven't personally discovered yet
As for 3D, it can be complicated, but there's a really good tutorial series by dragonitespam on youtube, it's really comprehensive! Most of how this game works was possible due to those videos.