1. I think for such a thing to be useful in a social setting the target needs to know (or at least strongly suspect) you did it. To do a more spooky thing to make the person paranoid, you could just do multiple things (like moving their coffee mug, opening things they just closed, and a bunch of other things while their back is turned).
2. It's a maneuver that applies the aspect "You Just Got Slapped!" on the target. But you said it makes a loud clapping (slapping) sound and impacts the person's face like they just got slapped. So it doesn't seem subtle.
3. Any aspect can be tagged for any purpose that makes sense.
Let's say in one scene your character got jumped in an alley and two thugs end up busting your leg (moderate consequence). The big boss that ordered the hit could invoke the consequence in a social setting to something about your incompetence or getting too old.