Combat rules assume the defender is more or less aware of the attack and can somewhat defend themselves.
I don't think I've seen differentiation between aware vs non-aware opponents in the RAW. Where do you see this?
I don't believe there are coup de grace rules. There is a narrative rule of "players don't die immediately when knocked unconscious if their WT is exceeded unless the GM thinks it's appropriate."
Really to be quite honest, I hate hate hate little specific spot rules for random game occasions. Nobody remembers them, it interrupts the game to look them up, and they come up constantly in other systems. I prefer to let the "preparation" rolls like Stealth add Boosts or maybe an Upgrade to the resolution roll (i.e. combat). If you're a Stealth god but only fair at melee, your Stealth roll could easily generate a ton of good die results to boost that melee roll with bonuses and upgrades. It's quick, clean and most importantly uses the existing rules framework.