Thanks for the feedback!
I agree about the tank shapes not indicating front/back well enough. The art assets are from a free asset pack from Kenney.nl, but at some point I think I will modify the tank sprites in some way to make them clearer.
The paste not working in Firefox is a known issue that I haven't managed to figure out yet. It works fine on Chrome, or with any of the downloadable versions for Windows or Linux. I suspect it's a Godot bug.
Your UI suggestions are great! The "players" being modifiable is weird and easily fixed, and the "X" button does really act as a back button, so that's a much better metaphor.
I'll add all of these to my TODO list. :-)