For example, Zombies get light, so have a 25% chance of negating extra damage, because headshots are generally easier to get than, say, stabbing through the heart for vampires.

Or, just make it a flat 50% fail chance.

(What do you call people from Ottawa?
Capitolists!)

In any case, that’s not who we’re really concerned with here. It’s the crit-immunes we care about right now.

As for math vs rolling, I don’t disagree completely. If we were to make it a flat +5 (or something) to confirmation AC, it would be just fine (though, to me, grabing a second d20 and rolling both is quicker, it’s not going to please everyone). What I dislike is when every creature needs a different value for crit resistance. Zombies have +4, Vampires +2, Genatinous Cubes +10, etc. Even if we make it something like 1/2 CR or 1/2 HD, it requires every creature to have a new entry on its stat block.

So, yeah. Flat bonus for crit resist = good, simple, and quick.

Roll two dice, use lowest (effectively you need to confirm twice, but you only do it once) = also good, simple, and quick, but different from most 3e mechanics, so some people will be resistant to it, of course.

Primarily, though, the second roll is just the way I prefer. No extra modifiers to remember, no players wondering why the AC suddenly went up (and why the golem’s AC went up more (or didn’t go up more) than than the zombie’s.

I think there already is crit resistance. You even mentioned it. Confirmation roll combined with AC.