Sliding Genioplasty chin bone healing question?

10mm is a lot so be careful.  Usually, lengthening genioplasties only will lengthen and project a few mm out and down, but it certainly depends on the situation (without photos, it's difficult to determine).  If you truly have a vertically short chin, I agree with the genioplasty approach, as while you can lengthen the chin with an implant, I personally don't think it looks as good (and I'm a surgeon who does more implants than genioplasties---the main reasons I currently perform genioplasties is for vertically short and asymmetric chins.

the increase in the height of the chin is usually not of the magnitude you noted. However, in any event, the 'intermediate' 'space' is either left alone to fill in (like when you have a tooth removed) or filled with  bone or 'artificial bone material'. the area is very vascular and will heal properly without concern. It regains good strength.