I am considering a brow bone reduction. How common is hair loss and scar widening on coronal incisions?

Brow bone reduction can be done via a variety of incisions including coronal and trichophytic incisions. Choice of incision will depend on several factors. Either way, if done well, the incisions will heal as a fine white line and hidden by the hair. Of course there is a risk of the scar being too wide or hairloss especially if not done carefully. Best wishes.

Hair loss is not a common complication of forehead bone surgery, especially in young men unless they are actively balding

Hair loss is very, very rare secondary to brow bone reduction. I prefer to do this procedure with a hairline incision where the hair will grow through and in front of the hairline, avoiding the more invasive coronal scalp incision within the hair. If you consider this approach look carefully at the hairlines of your surgeon's patients to make sure the incision is not visible.