I would not modify the damage according to pace. If anything, I would modify it according to speed instead. The way you compute it, any charge at a given pace has the same multiplier, regardless of the charging speed. But if a given pace is twice as fast because the mount is naturally twice as fast, then the kinetic energy transferred on impact will be twice as important. If you think about it, that's why arrows do more damage when shot by a bow than when thrown by hand.
The speed damage modifier would apply to all weapons that use kinetic impact, not just pole arms, If you want to nitpick, I would apply more to pole arms and less to weapons that are swung in an arc, but still, the only weapons for which the speed impact would be negligible would be missile weapons.
I would also remove the riding manoeuver. I would just limit the weapon skill by the riding skill.
If the attacker is on a large mount, they would benefit from higher ground.
The only way to properly counter-charge is if you have a shaft weapon, preferably set into the ground so that the kinetic transfer does not push the weapon back (forget about holding it in your hands). If the weapon is properly set and the attack connects, I would use the same damage modifier (speed of the attacker).
Longer weapon is of primary importance in a charge. There's a reason why infantry developed longer and longer pikes at the end of the medieval period. Longer weapon *automatically* hits first in this case (no relative initiative, just longer weapon goes first).
If you use weapon breakage rule, wooden shafts have a rough time in charges and counter-charges. Having the shaft literally explode in your hands is a distinct possibility. Most of the damage still occurs, however.
For attacking the rider from a standing position, I would simply use the higher ground modifier reversed.
I wouldn't limit charges to mounted protagonists, however. It is entirely possible to charge on foot, and I believe it was actually a fairly popular, if q bit suicidal at times, way to initiate combat when armed with a spear or other weapon and going downhill. It is obviously less efficient than charging mounted, because of 1) the difference in speed, and 2) the fear factor of a 1400 lb warhorse galloping towards you, but it could work.
Speaking of fear factor. I would have a morale check on NPCs that are on the wrong end of a mounted charge, unless they are trained in counter-charge and have an appropriate weapon. When the aforementioned warhorse rushes closer and closer, all hooves, teeth, possibly armour, and a rider with a spear on top, you really need discipline and training not to break. And even with those, not having a longer weapon makes you liable to simply be trampled to death by the mount instead of being impaled by the lance.
Do not neglect the trample - depending on the mount, it can become a much more significant factor than the threat the rider poses. Imagine an elephant charging at you and picture if you care more about the man on top or about the mount itself.
The drawback of charging is simple - you don't have much leeway in terms of defensive actions. Dodging, in particular, is almost impossible. You might want to *at least* half any skill bonus put into DB, and I think I would personally remove it entirely and leave only natural DB and shield.
