Chin vertical height reduction. Is this realistic?

Your approach is very realistic, proportionate, of very good tast and feasible... but feasible by few.You may find real trouble to find a surgeon who does osseous genioplasties... among this small group the vast majority will tell you they only have experience in sliding genioplasties to re-project the chin... which is exactly what you don't want and need.On your simulation of wished results (could not agree more with a patient request) you have done the 2 modifications your chin really needs:-vertical shortening-and, you did not mention in your text but have actualy morphed in your wish simulation, horizontal narrowing to make it less square and more feminineYes, it is feasible and I have done many cases like yours.If you wish better grounded opinion please do post or send privately well lit, focused and standard images: frontal, both lateral and both oblique views.
Feel free to request any additional information from me.

Through an intraoral approach your chin can be reduced in vertical height. What you are demonstrating is not completely realistic because you are not taking into account the attached soft tissues of the chin. The bone be reduced significantly in the vertical dimension but the attached soft tissues do not shorten as much as that of the bone. (since they are not being reduced as well) Thus a realistic vertical chin reduction is about 1/2 of what you are showing.

Changing the height of a chin is quite basic for Maxillofacial surgeons so start there. Your altered image appears to show an increased width of the angles of the lower jaw.  This is quite reasonable to review at the consult since longer face patterns tend to have more shallow angles of the jaw