How does attenuating a genioplasty step-off work?

There are many reasons why you would feel the screws. Maybe there was a minor infection and the screw didn't attach to bone. Regarding the chin, I would think just pushing back the chin would reshape it back to normal and wouldn't need shaving. X-rays and clinical evaluation  would reveal the situation better. 

The most likely reason you can feel a screw head now is that the tissues over time slowly shrink around the bone and metal hardware. When you have it reversed the fixation hardware needed is minimal and of very low profile. When you reverse a sliding genioplasty, particularly if it is completely reversed, the notch is eliminated by the bone sliding back into it. if it is only partially reversed, then some notching may remains which can be eliminated by burring or adding some filler material into the remaining notch.

The screw may be prominent due to the  bone 'remodelling' which naturally occurs after surgery. The screw can be adjusted or removed. The 'steps' can be reshaped or ideally, you could have a gortex strip laid over the area to achieve an even contour of the jawline; and potentially enhance the looks