Yeah, like you say, it doesn't matter at all, but it is fun to colour in the blanks.
After they run or something, they start to hyperventilate, which is fair since we don't want the actors to, well, die in the pursuit of continuity. The way I see it is... the body just keeps behaving more or less as before out of reflex. It sweats, it breathes, it's hair and nails grow and presumably it can digest food if need be. (Spike's weetabix. And God bless Jaffa Cakes by the way, Americans don't know what they're missing) But the demon prevents it rotting or the blood congealing, and the intake of blood (and with it, life)just keeps resetting the corpse to more or less to the way it was when it died (like Spike's spine repairing itself) So the breathing doesn't serve any real function, but it's a hard habit to drop, and useful for speech. And since the body is essentially frozen at the point just after death, the nerve endings should still be live.
What would frustrate the "Reset" idea is vamps with scars. Spike's scar in his eyebrow, and Kakistos' eye were both received after they became vamps. The only way I could work around that would be if the wound was inflicted with a special weapon, or by a Slayer's hand or something, but that's getting into very speculative territory...
And by the way, how does Angel shave...?
