In Game Master Mode (or within the editor), would it be possible to customize armor and creatures to eliminate armor hit points/shields (or reduce them to a large degree) and in their place assign various resistances, such as physical or magical to reflect armor value?
You can ensure that every single monsters/enemy in a campaign as ZERO armor/magic armor, Yes.
But the moment someone cast like, fortify, they'll get armor.
GM Mode is using (and will always) use DoS2 core ruleset as Swen said. This means that you can add all the resistance that
are in the game right now as you like.
Can the Game Master edit their players action points in the event that a player wants to do something outside of what the system normally allows in combat, such as talk to an enemy during their turn or grapple with it, or whatnot?
You can add/remove AP on the fly to any character.
