I can't reproduce multiplying score bug, nor see the reason in code. Which screen was affected? With flavour victory/lose text, or with high scores list?
I added quitting game by "X" button, but it's going to be some kind of panic button (close immediately, save game) for now - until I'll implement submenu for handling this case.
Actually, there is no progression - except starting and final levels, the order of carriages is random.
And introducing progression through weapons is not likely. Don't get me wrong, I see the point. But it can't be done easily due to basic game mechanics. Enemies have only 1 HP, so I can't just buff weapon damage. There are other properties (accuracy, effective range), but I can tinker with them only a little - five shots off target in a row would be opposite of fun ;) ...on the other hand, maybe I could add new mechanics, like sawn-off shotgun with cone area-of-fire...