Attack registers whether the soldier hits or not, lethality registers whether it's a kill.

Considering how slow horse lances are able to punch a hole in any unit, low attack and high lethality will usually trump high attack and low lethality.

Shields count as a defensive value in melee on the front and left side (but not the right, where it's skill and armor.) Armor counts against missiles. I don't have the info to help you with the ratio of shield vs armor

Yeah, pretty much, they all defend on different sides of the body (shield left and skill right. Both in the front), with only armor being omnidirectional

I don't have the link for you, sorry