I honestly probably would have handled Covering Fire etc with an Intimidate check rather than an attack roll - since the whole point is that you're trying to get enemies to keep their heads down. If you fail to convince an enemy that you can actually hurt them, they should just ignore your covering fire attempt. So in this case, even if playing by rules as written, I'd probably give an incorporeal target a large bonus to their "effective AC" for covering fire, to represent it being less likely to think you can hurt it.
Hmm interesting. I’d say that in the case of DR a creature couldn’t be certain that an attack wouldn’t eventually damage given the randomness.
Maybe to make RAW work, you can just imagine that the damage it’s invulnerable to is still painful and it would still want to avoid the attack