first off, thanks for taking the time to write all that! It helps me out a lot!
About a tutorial, I definitely intend on making a detailed ingame tutorial in the future. For now though, I think I'll do what you suggested on having an image describing how to play, at least.
About monster behaviour, their (current) attacks are indicated on the bottom icons, hovering over them showcases their range (or specific area that will be hit), and their number represent the damage they will deal. Though that is another thing that needs to be explained in a future tutorial.
I have a bit of resistance when it comes to making an ingame encyclopedia... Feels to me like it could be a waste of dev-time on something that most people won't bother to check. Still, I think you raise good points, I'll consider it for the future.
One last thing, yeah audio will be added, I don't plan on the game being silent.