So sorry it took me so long to get back to you—I've been working on a few books I've published over the past few months and have spent terribly little time on Itch.io. It should be relatively easy to add a way to quit the game, I'll look into the issue with the L key, and I'll look into the mouse issue.
I've had a few projects piling up on Itch from a sequence of game jams a half year ago; I've been trying to decide which ones need to be brought to completion. Flatland is certainly on that list. When I'm done with my third book and back to working with Godot, I'll make it a point to review these problems.
Thanks for playing!