Still, my point is, i don't think it messes up the balance to read it as written and call "totally at the opponents mercy" helpless.
What are we balancing?
The Coup de Grace is obviously the game designers creating a game mechanic whereby you can kill someone who is "helpless" in one round without people have to hack at a creature like a butcher. Clearly they did not intend for this to be a mechanic whereby anyone could go around slitting throats on the unsuspecting.
Your question juxtaposes "completely at an opponent’s mercy" with "totally unaware of your presence" and seems to be asking why those shouldn't be considered the same thing. I think the short answer is that in the case of the former, you offer no resistance to the coup de grace..whether you can or cannot. In the latter, you are able to react as soon as you become aware of the attempted coup de grace and WotC has seemingly decided that this foils the attempt...universally.