[Edi]

Damage is open-ended. Damage bonus actually increases die size. The reason why it appears as a bonus is because a staff can be replaced by a better magic weapon and the damage bonus would then increase the die size of that weapon. For monsters with natural weapons, strength increases (from spells etc) directly increase die size of weapons.

Armor doesn't get die rolls added, it's a direct subtraction. The mechanics are a bit more involved than that, but it's all going to be in the manual.