I think you're logic here is flawed... because the fact that a mundane person dropping off a ledge was able to succeed then logic stands that a projectile far more dense and released at higher velocity would likely also work. As to the possibility of it... unless teh ship's crew was entirely deployed on the ground and forced to rally back up on the ship it would make sense for some forces to man the guns... at teh very least such crew would be needed in securing more time for ground side forces to disengage... That just makes sense.
The horde position is irrelevant to the goings on of the ship with respect to what actions they could take. The Sky ship was manned in some fashion, crew keeping her active and in the air to respond to commands... all the sky ships have guns below deck level with the gyro blades it uses to maintain altitude...
yes... and it's form was basically point blank with the guns. It wasn't a small target... a near full broadside at point blank would likely do more damage to the fel reaver than Varian diving for a glancing blow.
edit:
if you rewatch the cinematic you have Mekkatorque call in the Gunship... it swoops in GUNS BLAZING (indicating that yes there was a crew and it included manned cannons). The Fel Reaver grabs the ship's top deck and pulls it off level, an act which also conveniently should line the guns up for a broadside on itself.