Definitely get 32 endurance augments. It is ok to have some defense rating in your gear, but if you're min maxing you should eventually end up with just shield and absorb rating mods/enhs, since shielding works against weapon (white) and kinetic damage, and defense rating only works vs weapon damage. Hopefully defense rating gets buffed in the future.
As far as the crit issue... critting and shielding are determined simultaneously. If you have 30% shield chance, and they have 25% crit chance, then there will be a 30% chance of a shield, 25% chance of a crit, and 45% chance of a normal hit. The attacker's crit rating doesn't impede your ability to shield unless their crit chance > (100 - shield chance). Basically, an autocrit.