Thank you for your comment and I'm glad you enjoyed it :)
You are correct that the 6 upgrades winning condition is an afterthought. Initially I wanted the game to end by cleaning all the garbage, but it felt way too long. I imagine it as 'you've proven to the school principal that you're great at collecting garbage' reward. If you have a different suggestion for a winning condition I'd love to hear it.
For your questions
1) Nonono. Would be even more random, more punishing and the stage steps would need to be rebalanced to allow more explosions.
2) No, only for rolls from then on
3) Just one time. It's indeed not clear in the rules.
4) Good point, I didn't cover it. No, garbage held aren't added to the total, only garbage on the board.
What did you think of the 'Protect adjacent squares' upgrade? Did it prove useful?