wireless networks alone aren't going to be sufficient to build a large-scale mesh network that could disrupt existing ISPs. The latency alone makes this infeasible.
I'm not so sure, your links are almost c, whereas optic fiber is substantially slower, so it depends entirely on how little processing you can get away with in the message passing code.
edit - was just thinking about this, an arm cortex runs at ~ 2,000 MIPS, which gives you just under 15 cm per instruction for radio in air. Fiber is about a third slower, so your links do not actually have to be massively long to give you quite a few cycles to play with.