2) A single Pi can handle up to 32 USB devices (thus, 32 Gridseeds), correct? Will the software support this number? (I switched over to cgminer for more accurate reporting)
I can't tell you about the ZH image, but assuming the limitation is the pi itself I saw it max out at 18. The pi could barely do anything else so checking the hashrate locally was a pain. Luckily the pi is cheap and takes nearly no power, plan accordingly.
I could be wrong, but i thought max is 32 usb device for pi run stale if you dont use cpuminer. running 32 instance of cpu miner in a RPI wont be a stable IMO