Sliding genioplasty incision tightness, numbness of lip and chin - Would reversal put things back to normal?

A 6 mm horizontal change is not an excessive amount of advancement. A genioplasty can be reversed but it is unlikely to change  the degree of numbness. In order to minimize the tightness and scar contracture the position and type of incision is very important. An incision made along the necks of the teeth with a small release near the canines will provide access to release the scar tissue. If the chin  position is aesthetically acceptable the plate and screws can be removed and any small steps in the area of the mentolabial fold can be re-contoured.  Good luck

In sliding geniplasty problems like yours, in which incisional tighteness and contracture down into the step off of the bony advancement, release of the lip and labiomental fold with interpositional fat grafting would be the appropriate approach in most cases. This could be combined with subtotal or total bone reversal if there are more generalized issues of overall chin pad tightness that extends beyond the lower lip, vestibule and labiomental fold.

You need to discuss your concerns with your surgeon. That's the person that knows exactly what was done.