Thank you for taking the time to play and review my game! I decided to go through and address each issue you presented in an update:
I addressed keyboard input, squashed the transition bug, balanced the movement speed upgrade's price, added a change to the upgrades menu when all upgrades are maxxed, implemented an easter egg which can be found at the bottom of the mine ;), and changed/added a few aesthetic things.
To address point 2: The combination of the jam's theme and limitation immediately made me think of those upgrade-based Flash games, so that's the style I decided to roll with! I completely agree with the way you view my implementation, but here is the way that I justify it: much like the drill's mining efficiency, its "health", or fuel capacity, is a tool that you can upgrade in order to spend more time in a run, thus maximizing profits. I believe that is may become more evident once I add more ways to take damage, such as explosives and enemies.
To address point 5: I did not realize that I made that reference, but I see what you mean! Perhaps I should change the values of the gems in order to solidify the reference, and then pretend that I meant to do that all along :). To be honest, the real reason I chose red, green, and blue for the colors of the gems are because of the RGB color model!
To address point 6: lol
Thanks again for playing and going so in-depth with the review!