Thank you so much for your praise! I'm very glad you liked it.
Indeed, I was using the 'forward design' strategy most famously used by jblow. I'm blown away (hah) by how quickly this strategy leads to decent puzzles.
I can actually explain what's going on in Canyon: [8/] [3] [/3] takes two turns to collapse to [8], similarly how [+] [+] [2] takes two turns to collapse to [2]. It seems division is left-associative i.e. "8/3/3" is interpreted as (8/3)/3.
But the fact that you didn't understand is totally my fault. As with the -10 thing, there are some concepts I should have introduced earlier in the game via simpler levels.