I agree with OP. Horse charge should be decreased. This would also solve some of the issues with bumpslashing. Hyena I think you used the wrong formula there. I think you are talking about inertia. P =m*v mass times speed. F = m*a yes, but thats mass time acceleration, which proves that horses must have powerful legs to accelerate as fast as they do in M&B even though they probably weigh like 300kg or so. However, your argument is completely correct. When something with a large mass comes at something in high speed, it has a lot of inertia (and also a lot of kinetic energy (Ek = 0.5*m*v^2)), which means the poor infantry will get a lot of inertia and kinetic energy transferred to him. Unfortunately for the little guy, there will not be a perfect elastic collision (in which case all the inertia and Ek transferred to him would be in the form of his speed increasing, most likely backwards) no, there will be a partly inelastic collision which means some of the inertia and Ek will be tranferred to knocking the man down, while some will go into his body and break his bones and such.
So, if a horse charges into a man, the man will (in reality) be damaged quite a lot. However, horses dont like to run into people, because it hurts for them too. They get the same amount of "internal damage" as the one they hit, though as mentioned the horse has lots of bones and muscle, but still not so chill for the horse. Only way to get horses to charge into people is to have a pack of horses get afraid. Or train them to do so, like the heavy armored horses have. (but not the light arabian horses) (and the armored horses have armor on them, its better to damage the armor than to damage the horse)
However, I agree with the OP for balancing reasons. Bumping with a horse is a cheap way to fight, and as light cavalry you're not supposed to charge into packs of enemies with your horse. Thats the heavy cavaly's job.
When something with that much mass wallops into a shield, what happens is the energy transfers into the body of the shield user, which then makes the shield user fall on his arse and get trampled with very little trouble to the horse. A sheild, if anything, would make it -easier- to hurt you charging you since theres more area to transfer force from.
Just a comment on this one too: If you hold a shield up and try to stop the horse with it, you will most likely break the arm. But if you hold the shield just infront of your body, and only holding it up with the arm, it will defend you from the collision since the shield takes up a lot of the kinetic energy. (the shape of the shield is also helpful here) The size of the shield does not increase the force on the man, (you might be thinking about drag force or something) however, the size of the shield helps the horse from glancing off you if it doesnt hit you perfectly straight. If the horse just glanced you, you wouldnt be so damaged since the horse doesnt lose so much speed. (less Ek transferred) But with the shield you are almost guarranteed that the horse will get a proper inpact and blast you to smithereens.
EDIT: I just noticed the OP said something about not letting lances attack without couching. Thats bs, couching lances are boring. What is fun with being a lance cav is the difficulty with hitting with the stab attack. Couching is also the easy way of killing non pike infantry, not the hard way. (even though coucing takes more time to charge up to) If lances was couch only, we'd see a lot more cavalry with spears and such, or just have none lancing cavalry left. (except for the sissies who prefer easymode, no aim required, no timing required one hit k.o couching) (not to bash on people who use to couch, it requires more strategic sense, and I respect that)