FNavigationRelevantData.Modifiers is a composite of multiple modifiers, so yeah, you could have multiple volumes.
I can’t speak for everyone, but we don’t bother excluding unreachable surfaces from the navmesh. AIs won’t try to path there anyway, unless you request a random navmesh point that ends up there. Instead, we always filter our navmesh point requests to whatever surface type we want and those areas are valid.