What are the specific effects of hidden armor bonuses in Minecraft?
No, Projectile Protection does not reduce knock-back from projectiles.
Yes, Fire Protection's burn time reduction scales with the level. And there isn't a cap.
Explanation:
Protection: Reduces damage from most sources, nothing else.
Fire Protection: Reduces damage from fire AND reduces burn duration.
Blast Protection: Reduces damage from explosion AND reduces knock-back from explosions.
Projectile Protection: Reduces damage from projectiles(ex. fireballs), nothing else