What is the best sliding genioplasty option for a vertically shorter (a lot shorter) but advanced chin? (Photo)

Much simpler than all those technical proposals you've received; when the chin is not suffering a serious shortfall of bony mass and is:-receded lacking projection-too long vertically(like happens in your case, you have good eye)In such a case the solution is a jumping genioplasty.In the overlapping genioplasty you perform a complete horizontal cut
just below the mentalis nerves point of exit and through the whole chin
and its side wings, the mobilize the cut fragment attached to the oral
muscles, and finally overlap the fragment so that the back cortex of the
chin jumps onto / makes contact with the anterior surface of the jaw;
by means of this witty maneuver the chin is forwarded and at the same
time vertically shortened, which is what you need.See the link below to find few cases of my own practice very similar to yours which I had the opportunity to operate successfully on, sharing a lot of common features with the technical problem you have posted.
If you wish better grounded opinion well lit, focused and standard images have to be assessed: frontal, both lateral and both oblique views, also from underneath.
Feel free to request any additional information from me.

Although a full face view is not available, your Xray suggests that your chin and jaw vertical height are balanced for your face, but your chin lacks projection. Reducing your vertical height further may make your whole jawline appear small. Rather, a chin implant (or custom chin/jaw) implant offers the benefit of controlling your desired projection and contour in three-dimensions, without the need for bony work. In addition, it avoids any potential palpable step-offs along your lower jaw line that can result with some genioplastigs. This would be an alternative option to a sliding genioplasty and would give you more precise control over your final contour.Best of luck! 

I think you would definitely benefit from a substantial advancement in your genioplasty. This will camouflage the excessive vertical height of your mandible to some degree. Bone reduction can be done to decrease the vertical height of your chin. Continue in person consultations until you find someone you feel comfortable with.  If you don't want an advancement then you don't need a sliding genioplasty. The sliding part is for adding projection either inferiorly or anteriorly( forward or down projection) in my opinion you need some forward projection and reduction i.e. no slighting needed for inferior projection instead this is accomplished by bone removal. Hope that makes some sense but it is unlikely I'll be doing your procedure so it's best to have these conversations with doctors who will more likely be candidates for doing your procedure. Best, Mats Hagstrom M.D.