You really haven't captured the nature of HP, I think.So... I'm a human commoner who has lightning fast reflexes, and by lightning fast... I mean I'm literally "The Flash". I can move faster than the speed of sound.
But I JUST got my powers, so I still have the hitpoints of a level 1 commoner. We'll say 3 because of my sickly con because I did a lot of meth when I was younger or something.
Now, despite having an AC of 99 the Reaper can murder me without question. I simply can't beat him. If I'm standing next to him at the end of combat, despite being able to watch him swing his axe, eat a hamburger, drink a milkshake, and debate the virtues of Oprah's marriage... and then step out of the way at my leisure.
I still die.
GTFO. I could kill SUPERMAN if he had few enough hitpoints. He might have DR/- 9999. Doesn't matter. Superman dies. Don't even need kryptonite.
Now you might say this is nonsensical but stuff like this WILL COME UP IN THE GAME.
And the question is this: What is so damned important about keeping reaper the way it is that it supersedes the suspension of disbelief ire it will cause for many if it's kept in?
Is some vast wonderful thing gained by having it the way it is? No. Not really. Whereas by removing it, and things like it, you dodge a particularly nasty bullet.
It is not just AC which should be reflected from such speed. HP would also be reflected.