As stated, it's roughly 2 inches from the wall (or ceiling), or maybe slightly more.
Re whether you can mount a QS8 from the ceiling with FMB, I think yes. I mounted a VP150 on a 45 degree knee wall with FMB, and a QS8 is much lighter than that.
From a safety standpoint, it's probably important to hit a wall or ceiling stud (which I did). In theory there are some dry wall anchors that would hold a QS8 or even a VP150, but screwing into the underlying wooden stud is more secure.
It's also important to carefully tighten the allen screws on the FMB. You don't want a QS8 falling on someone's head!
Also don't take our word for it; check with Axiom about your intended mounting technique.
