Rereading Mirror Shield I think UG should actually deal damage through Mirror Shield because its not a spell, its a permanent. Things that poison (like deadly poison) don't actually deal damage so they shouldn't be reflected.
Imagine if Mirror Shield and Jade Shield
didn't reflect Unstable Gas. It's a direct damage spell in the form of a permanent, therefore it is reflected (...and obviously because if it wasn't reflected, it would be very powerful).
Catapult doesn't convert a creature into direct damage and then fling it at your opponent. It simply throws the creature at your opponent, dealing damage based on the size of the creature. How would a magically enchanted shield, built to repel direct damage, be able to reflect a creature?