I think that what Springheel suggested is still something to be seen as a compromise as it still leaves the possibility for "one strike takedowns".
Beneath the fact that it results in extra work it only leads to inconsistencies. Having this embedded as a fundamental part of the system makes more sense IMHO then laying this into the hands of the mapper. The absence of this possibility once an AI is alerted is just some sort of punishment. This means you can take out a hundret guards with one single arrow.
Actually you can even retrieve the used arrow from the corpse. If you do not alert any guards, you'll be able to take them out one by one. I think it is a bit overseen how rewarding the proposed system is. In addition if something works the one time and do not work the other time although the player does exactly the same it will only get frustrating to the player.
Also having some sort of randomness on the possibility of one arrow killing a guard will only cause the player to reload if it fails. If you engage in fight those fights won't end up the same every time because the player behaves different every time he tries it. I'm not a big friend of randomness I must say. It is not really about enforcing stealth gameplay, it should just become a bit more rewarding. I think that would be a natural way to "enforce" stealth. This is where stealth comes in, because stealth is the answer when everything else seems less of a reliable option. The more fatal and less predictable a system works the less abusive it is and the more "easy game" and "hard game" blend and cards get sorted differently. If you implement a system that is highly systematic, where you know they can't kill you as easy as you can kill them, or the other way around, where you know you die easier than them (always), than that allows players to adapt to it and at the end learn to abuse the system. Stealth is basically like playing it safe, but has it's own kind of risks and intensity and thrill (as we all know). What we create is a situation where the player can decide to go on bow rampage and the chances are really good to have an easy game because an arrow and a single sword hit can kill, mostly, but knowing that being killed is also as easy as to kill and still to win the rampage creates for a more thrilling and intense experience. You should rely on the power of a single arrow shot, knowing it kills, but also know that you can die by a single arrow as well. You should know that your sword can kill with one hit, so you should know that you can die in the same manner. To enforce stealth without to screw up the realism of the combat system is to make combat fatal and unpredictable, for both, the player and the AI. So you can call me a fatalist and realist. The third option is what Garrett prefers, stay hidden and don't even risk it, though this has it's own way of a risk, but you take less chances. Either it's risk and win good or lose hard. Basically by making it a random event you take away the confidence and security from the player and that makes running around and one-shot-killing everybody a potential risk that the player can't predict the chances of. All you can rely on are chances, a single arrow to the chest has a very high probability of a kill, but there is no 100% to it, the guy may end up charging at you with three arrows sticking out of his chest. A hit to the neck, throat, head is about 100% kill, anything else is up to fate. Yes, a single arrow can and should kill an unarmoured opponent instantly, but should there be a guarantee that it always does? No. Sorry for not reading all of the posts, but my opinion on the subject is that I prefer a fatal condition to the mechanics.