Is it possible to safely combine sliding genioplasty with a chin implant to achieve increases in projection of 20mm or greater?

You can combine a sliding genioplasty with a chin implant to likely reach about 15mms, maybe be a few millimeters more. Like all sliding geniplasties there is going to be some risk of sensation loss which in most cases is only temporary. But to skirt this issue completely a custom chin/jawline implant can be used that makes this risk as close to zero as can be done. In either case, however, such a large chin movement (no matter how it is done) will leave the lower lip where it is as the chin moves forward. This is why it is probably not the best aesthetic idea to try and completely correct the chin position to an ideal normal position.

No. I do not recommend downgrafting more than 4-5mm. Anything more than that would need a bone graft from your hip. Otherwise it would be unstable and not heal properly.

I would see an oral maxillary surgeon and probably would see the results from the genioplasty before committing to a chin implant also.