It seemed like the change could be summarized as, "everybody gets a chance to deflect, but melee classes also get some of skills that make their blows less likely to be deflected." Guess that's not the case after all.

That is the case. It's a bigger leap to say "largely negated by", which isn't accurate.

* Do all gear slots "matter"? For instance, is there a benefit to wearing AC-ish rings over "treasure" rings? It's hard to imagine deflecting something with a ring.

Nearly all slots matter. Most rings don't give AC (those that do tend to hit magic), and slot is checked very, very rarely.

* Do the "armor use" skills stack? For instance, if you had a really light piece of dragonscale would it benefit from "bone armor use" and "light armor use" both?

Neither. Dragonscale is neither light (cloth, leather, fur, hide, etc.) nor bone, and not a material I'd assume any adventurer gets trained with in a guild. The ability to use dragon armor exists, but isn't taught in a guild.

* Can I assume that "light armor use" will only help you, i.e. there's no penalty when especially heavy armor is worn?

That's true for all the Armor Use skills.

valguarnera@carrionfields.com