this kinda assumes that ac is all of the things about a characters defense that does not require effort for them to expend in order to avoid lethal attacks.
Interesting.
this would imply resistance and immunity gives you omniscience about sources of damage if they will or wont hurt you. cant think of an explanation for vulnerability though?
Are damage types all that obscure?
Especially if the resistance or vulnerability is natural the extra effort or lack thereof could be a reflex.
Like how you touch something hot, you don't have to determine it's not, remember that hot things burn and choose to snatch hour hand away, you just do.
The exception where you might expend effort to avoid an attack out of all proportion to the real danger might be psychic damage and illusions that inflict up damage.
if your implying that the fact that you are avoiding greater danger in the form of a higher damage has an impact on how much effort you must put into avoiding the attack then i think...now the very possibility of danger has a physical impact on a characters ability to avoid lethal injury.
No, remember: hps and the ablation thereof are not primarily physical. There's no inconsistency there.
additionally there's no explanation for how poison can be transmitted by injury if technically you cant be injured until you run out of hitpoints
Technically you can't take lethal injury until then. You could get a scratch or skin contact sufficient to transmit a poison that has a affect other than to damage... of course some "poison damage" that us just go could be because if the extremity of avoiding so much as a scratch.
nor does it explain how the hitpoints that poison drains from you as it kills you is drawn from the same source as the metaphysical force of damage possibility reducing your characters endurance.
Being poisoned could easily erode your endurance.
Remember, you can briefly say "hps are endurance" or something as a shorthand, but it's an abstract mechanic that could represent any factor that helps you avoid or minimize the effects of what would otherwise be a deadly attack (and, I think, it helps to remember that, for instance, a dagger is absolutely a deadly weapon, but only gets a d4).
By the same token, bestowing any such factor, not only restoring endurance or literally healing a scratch, could reasonably restore hps.
Since the DM has a free hand in narrating results, and the mechanics if combat are so abstract, he can choose to keep the narrative in line with the mechanics (or vice versa) to the degree he's comfortable with.