1. This I have no idea. Maybe the type of weapon you wield effects your dodge in the first place.

2. If you're dual wielding, the game randomly picks one of your two wields for you to parry with.

3. Dual wield checks that the weapon can parry in the first place, and if not doesn't consider parrying with it. So you only parry with the weapon that can.

4. 2H parries better than the same weapon not 2H.