Realistic, huh? Would you rather take sword or wear plate armor to survive longer against competent opponent? Forgotten realms forgot about something as basic as that.
Anyway I think that taking complex real life events and interactions, bending them to fit into game engine and work according to precise numbers is herculean task with pyrrhic result.
Regarding precise amounts:
I agree with potential minor WP penalty, I dont think that another movement penalty is needed, armor should have its weight precisely stated and thats all, warband calculates your speed with that.
Missile damage isnt close to good, what happens when i take weak bow and use noldor arrows with that? Force of impact is indeed true, but its heavily dependent on the missile speed. I can shoot the best arrow ever made, which can pierce modern steel, but I dont have the power to accelerate that so much with my own hands. Give something enough speed, and it can one shot kill heavily armored knight in full plate, it doesnt even need better material or high mass to pierce his stuff. I think that for missiles it should go like this: If bow has X dmg or higher, X PD requirement or higher, with X arrows or higher, you can create some kind of function, to calculate the effective distance of "blunt force of impact" bonus. If arrow had to travel 200 meters, it shouldnt do much against heavily armored target, I wouldnt be concerned about that, BUT if arrow was shot literally in the face, I would at least try to bow, to hide some weaker parts and holes and use top of the helmet and other parts to "tank" it. Check some kind of analysis of battle of Agincourt to understand what I mean. They werent concerned about arrows piercing their armor, it was all about the eye and breathing slits/holes. Arrows did occasinally pierce some parts of armors, but it just got stuck and didnt harm the man inside, at least not enough to seriously wound or kill him. Shooting from very long distance is a waste of ammo. If something like that was implemented in warband, it would make those "hold your fire" and "fire at will" commands very useful and important. But micromanaging this... ...is another story, and I wouldnt want to do that. AI is stupid enough, prone to abuse, this is another way to abuse it, or at least make it more annoying for the player.
So for now we are in this time, when compromises have to be made and some things must be contractual to make the game enjoyable. This is a big project and requires lots of dedication. If I may, I would like to suggest asking historians, museum curators, HEMA instructors, etc. about the missile vs armor argument, ideas to bend it into the game, by gathering informations about actual battles and remaining equipment from the period.