Apologies for the delay in responding - been a crazy week!
It sounds like you figured it out, but feel free to ask if there’s anything else that’s confusing. Happy to provide my thoughts.
Ability mods don’t come into play when defending, but if a creature has any amount of DR (from items or effects, etc.) they gain a +X to the check to defend, where X is the DR.
So for example, if a monster’s attack is 3, normally a character has a 3-in-6 chance at evading the attack. But if they’re wearing DR 1 armor, that becomes a 4-in-6 chance at evading the attack.
As an aside, I’ve been working on a few house rules regarding morale, sundering armor, and some other stuff I was planning on doing an errata for in the coming weeks, so keep an eye out for that!