It is a huge nerf. 90% of cavalry use unarmoured horses, and if they were killed or disabled in one shot (as you suggested in your post) then cavalry would never be played because you'd be forcibly dismounted by archers every time you spawned on a decently populated server, effectively turning you into a sub-par infantryman. As it is, cavalry are balanced. They rule the open fields unless you have a spear or good timing with a sword and they're helpless if they get caught in a mob or run into an obstacle.
Cavalry are, and should be a threat. If dehorsing a horseman was as trivial as shooting one in the head then cavalry wouldn't need to be considered when playing tactically. If you're getting run down by cavalry then get a spear or get in a building, behind an obstacle or an elevated position. CHANGE THINGS UP. Those suggestions seem like nerfs someone who takes on cavalry on flat, even ground in the open as an archer or infantry without a spear would want, as if you play tactically to avoid or dispatch cavalry the horses are weak enough to take out in two good spear thrusts or at the most three arrows from a decent bow.
Chances are if you're an archer and you don't have time to get in the second or third shot before he lances you, it's his kill. He had every advantage, and you had none. as an archer, it's your job to make sure you're covered by infantry or at least in a position where you can sit back and fire at horses undisturbed. If you're in a position where you can be lanced from any direction at any time, then you need to either group up with some spearmen or get an elevated, covered position.
To summarize:
Cavalry vs. archer in cover = Dead cavalry
Cavalry vs. spearman = Dead Cavalry
Cavalry vs. anything while stopped or up close = Dead cavalry
Cavalry vs. unaware or ill-equipped infantry = Dead infantry
Cavalry vs. unaware and vulnerable archer = Dead archer
Cavalry vs. aware opponent on open ground = 50/50 chance
That is balance. Cavalry only kills you if you let it.
