FaberCastell 1) I have seen forum members post here about not using their home wifi for initial set up - why is this important?
The idea is to prevent network providers from associating the device with your identity. It's more about cellular than WiFi. Cellular networks will see your location + various device identifiers as soon as you boot (even without SIM). WiFi is less problematic.
Whether this is important will be up to you.
2) Can I install GOS without first booting the tablet with its stock Android OS? I believed I had seen that mentioned here, but the 'Install' page says I have to enable OEM unlocking in settings, which presumably means I need to boot Android first?
You are correct. You have to boot & connect to the internet to unlock.