Apparently, there is a great deal of disparity in surgeons ability to re-suspend the mentalis in a position that is both functional and esthetically successful, particularly after large advancements. Pick your surgeon wisely!
Have you considered genio vs implant?
My surgeon has apparently done a number of genio's that were 8mm+ with a least 2 being 10mm for either sleep apnea or cases such as yours where jaw surgery is contraindicated. I had a 10mm genio done 2 years ago in an attempt to help my sleep apnea without having to undergo MMA (which I underwent 5 weeks ago).
My mentalis was stretched a great deal after the genio but I had no issues with chin ptosis afterward. I did not have any hx of pre-existing mentalis strain, however. Though we did it for functional reasons, I certainly benefited from it esthetically. My apnea was improved but I eventually regressed back to severe apnea as my tissue laxed--so however tight things are after surgery, you can likely expect a great deal of "loosening" over 6 months to a year, if your experience is anything like mine. I have also heard this from others.
After my MMA, where I was moved another ~10mm forward, my mentalis looked ok immediately post op, and then at week two, when we changed my bands, my mentalis began sneaking south and I began developing progressive chin ptosis. After both of these surgeries, my tissues were so tight, I could not look up. Strangely, my chin actually looks somewhat better than it did at week two but with certain facial expressions, I do have some degree of chin ptosis--the fat pad of my chin and the lower muscle bodies of the mentalis will sit just on or below the tip of my chin bone. AT this point I'm so swollen from a procedure 2 days ago that its hard to tell where I'm at in terms of my chin at this point. AGain, things seem to stretch/adjust a great deal over time after surgery.
Also consider that with bigger advancements in this area, you risk a greater likelihood of nerve damage. After my 10mm genio, I ended up with some permanent sensory loss in my bottom incisors and right in the middle of my lower lip (only 3-4mm across and not full loss of sensation--I had about ~70%). My sensory loss did not affect my life in any way really--kissing, eating/drinking, speaking were all good to go.