1. There was talk of varying cavalry sizes based what type of cavalry they were, but it was decided against. Just use less units of a certain type if you think you have too many. Besides, I think cataphracts would still be in large groups even if that plan was implemented.
3. Spears get a hardcoded -4 against infantry, so they are given extra attack to counter that.
4. A charging spear propelled by a horse should be able to pierce any armor. It is intended for just the charge but cannot be specific to it...
5. Because leathality cannot be modded for missile units, missile variation is very difficult. The AP on slingers represents the blunt trauma of units that use lead shot.
6. I doubt horse archers would have been divided in such a way...

I'll let other answer better...