Can a sliding genioplasty be partially "undone" (slid back the other way) safely?

There should be no problem to do sliding genioplasty again. It would be in your best interest  to discuss this with a surgeon who performed your initial surgery, if possible. Good luck. 

It is possible to perform a second surgery on the chin to improve aesthetics. I would perform a CT scan of the chin bone to evaluate the previous surgery. There needs to be adequate bone support to allow bone to bone healing. I know that you expressed reluctance to have an implant, but in this case, if all you are seeking is more protrusion a chin implant is the easiest solution. It can be placed through an oral incision and there is minimal recovery time. Good luck!

A sliding geniplasty can be partially or fully undone. Now that you know how the bony movements have effected your chin shape, you will have a better understanding as to how to create a partial reversal. Moving the chin horizontally back, as you have discovered, rarely produces a favorable aesthetic change.