How would a sliding genioplasty affect my open mouth smile? Is there a risk of a “witch’s chin”? (Photos)

Give it second thoughts, there is no point or need to increase your chin's projection, it may lead to a bad over correction and in your case, unlike other types, won't correct the neck's flaccidity.

A genioplasty is possible but you could also get, for much less cost and recovery, a chin implant. We do these through the inside of the mouth, they are customizable, it takes about 20 minutes, and you have very little recovery or discomfort. Plus, it's readily reversible.

Whether genioplasty or augmentation with chin implant should not change patient’s smile. A series of better photos would help to see whether you are a candidate for any of these? Neck issue may be related to other causes, not necessarily small chin. In person consultation with a board certified plastic surgeon with experience in chin/neck surgery would help. Good luck.