I will outline my understanding of how combat works based on observation and reading the manual.
Non-shield:
1) Base attack value is modified by attack modifiers; base HP is modified by defense modifiers.
1a) The modified values are displayed in combat details
2) Modified attack is compared to modified defense, via the (0-to-attack-value) vs. (0-to-defense-value) roll.
3) If the attack roll is higher, type of hit (kill, retreat, pin) is determined. Otherwise, it's a miss.
Shield:
1) Base attack value is modified by attack modifiers; base HP is modified by defense modifiers.
1a) The modified values are displayed in combat details
2) Modified attack is compared to remaining shield absorbtion, via the (0-attack-to-attack-value) vs. (base HP - damage previously absorbed).
3) If the attack roll is higher, the hit is a kill. If it is not, it is a miss, and the roll is subtracted from the remaining absorption.
Assuming this is correct, all defensive modifiers are irrelevant to shielded infantry, as the damage that can be absorbed is entirely determined by the unmodified HP. Looking at combat details and how shield absorption is progressively displayed upon a unit taking damage, it certainly appears to start at the base HP, and the modified defensive value (which is displayed in those details) plays no roll in combat outcome. While shielded infantry will not suffer from the risk of really bad rolls when first hit, they're overall far less attractive than normal armored units if they can't benefit from any defensive bonuses in addition to gradually becoming weaker over the course of an engagement.
Example screenshot of details included, although the above discussion is probably clearly than it.
