I got a tutorial message during that particular fight telling me to switch on non-lethal attacks.
No, that didnt happened to me ... if it would, i wouldnt run around the room trying to find some clue ...

But as i said, since i turned in on to knock out the merchant ... maybe that tip didnt show (or hid really fast) bcs i allready had it activated.
If you use ranged or spell attacks they'll still die with non-lethal attack toggled on. It can only be used with melee attack
I see how is this information relevant to anything in this topic.
But yeah ... i know that.