15min with a sharpie, a drill and tap says any 2U case fits as many ITX systems as you can physically fit in the space. Then probably 2-3 hours working out the wiring so the redundant psu's power them all.
Why bother with multiple motherboards when you can just have 1 and many VM's?
Wireless backhaul works fine if you put the extenders in place before the WiFi turns to trash (roughly neg 60-70db).
Pretty much everything plugged into the outlets bothers Powerline. Refrigerators, microwaves, ovens are probably the worst. Even using outlets on different breakers bothers it.