I'm no expert but I think your first set of "surround" speakers should go to the "surround" output. "surround back" would be speakers almost directly behind the listening position.
check out https://www.dolby.com/us/en/guide/dolby-atmos-speaker-setup/7-1-4-setups.html
from the same website.
The foam will make the wall assembly stiffer and more echoic. At least that's been my experience. Don't know how important one way or another that is to Cecaa in his garage. Acoustic treatment of course is still possible if wall space is available.