There are gray fortresses regardless of the number of players, so some missions don't use all possible fortresses by design. But what I meant to address are fortresses that are either red or yellow (some missions have two enemy factions), so I'll update question 7 to make that clear.
OK. While in co-op, I've only noticed few spawns on a fortress (never empty) or captain pop-ins near its outpost. Maybe later on.