Hey! Thanks so much for testing the demo.
First off, the screen issue when returning to the main menu is actually a browser-specific bug—it doesn't happen on the Windows version. To be honest, I haven't had the time to fix that one just yet.
Regarding the lower attacks for the Mermaid and Triton: I'm working on it! They really are the hardest to deal with because they're super fast. You're not a bad player at all—several people have brought this up, so I definitely need to balance it out.
As for the shop bug, that's a strange one—it's the first time anyone has mentioned it! If you tried to equip an amulet without having an empty slot, the game asks you to replace one of your existing ones. In that state, your only options are pressing ESC to cancel or Space to replace. Maybe that got a bit confusing and made it feel like the game was frozen? Let me know!
Thanks a lot for your feedback, I'll definitely keep all of it in mind.