Hm... Okay, I'm assuming by "mains networking" (I'm not familiar with that term) you are referring to powerline
If you can get an electrician to go over a few of the runs in the buildings, and to reseat the grounds in the main panels that should get the mains (/powerline) networks to function within their own rings. Then connect the rings with much less network cable and run the wireless access points off the powerline (/mains) network.
Basically run the network cable through whatever space is the easiest, and then branch vertically with the powerline units to position the WAPs for maximum coverage.
Our office used to be a small shopping center years ago...so the wiring here is all chopped to hell. But I had many successful test runs with powerline boxes after tinkering around in the panel a bit to get the grounds working properly ... So it might be worth a shot.