They definitely stack on my end- I did some testing with a Super Powder of Rock and 1 vs. 10 Stone's Throws, and an Autoreaper took significantly more damage. (EDIT: Yep, they stack, for sure. After taking off amps, 9 more Stone's Throws took a Super PoR against a Blocking Wood Scorpion from 17 to 71! Increase of 54, so 27 total attack added as intended, since they're weak to Earth. That'll really shake things up.)
I'll look into the Spite Smash issue later tonight.