I assume that both the main player and co-op player would have to start a new community, but is the co-op player allowed to loot in their own community before joining the main player as long as both games start at the same time and video footage could be shown for both players? Or, does the supporting co-op player only get to work with the three characters provided on community generation?

I would say no. The co-op player would need to rely on what's in the main's instance.