Thank you very much for playing and the very detailed feedback! I'm definitely going to note it all down. The first 3 bullet points make a lot of sense.
The fourth bullet point is the difficult one. The ability to change levels definitely needs to be clearer, as you said. I don't remember the original snake game being beatable on the old Nokia phones. But maybe if a snake of a certain size is grown, then the user can unlock the next level (but I think most people will want to keep playing until they die, so they can record a high score). Or there could be a level select screen.
Versus mode with human opponents in local multiplayer sharing the same keyboard should be perfectly fine. My game engine doesn't support online multiplayer (but maybe in the future). I think this mode could have a character select screen (also for selecting computer / human players), then a level screen.