I've taken a look at an appropriate head posture profile photo from your RS series, used some anthropometric measures of average 'real life' distances as to approximate a SCALE. I found the HORIZONTAL displacement measure your doctor stated for CHIN about 90% correct and not about 50% correct as Eppley is basically saying when he says approx 2X as much augmentation of chin is needed. What Eppley is NOT telling you is that the NET lower facial profile imbalance is from BOTH chin AND jaw recession. Eppley is telling you what he can do which is an implant chin augmentation that EXCEEDS how far the chin itself should 'stick out' to COMPENSATE for mandibular recession.
The other doctor is also is just resolving his answer to promote what HE can do with chin augmentation. But since he IS a maxfax, he hasn't bothered to tell you about 1/2 of the NET recession is attributable to the mandible. Not just the chin.
In the event you DON'T REALIZE that what you are asking for is a LARGE chin augmentation to COMPENSATE for TOTAL recession you have where about 1/2 is from the mandible and the other 1/2 from the chin. That's something that won't look AS BALANCED as would a BSSO in addition to genio. Only people with little to no chin recession but with some mandibular recession can mask the mandibular recession with a genio, usually the type that goes outward and downward. But, when chin augmentation ONLY is used for BOTH combined mandibular recession PLUS chin recession, it can look unnatural or just weird.
As to whether you can first have genio and then get an implant over that, the answer indeed is YES. However, using a chin augmentation to compensate for recession to mandible in addition to chin often looks 'off'. (Crescent moon chin).
Your doctor is just going to address chin recession. The reason the horizontal displacement does not seem like 'enough' is what you are seeing is not only chin recession but also jaw recession.
By the way a SLIDING genio to bring chin 'forward' has 2 displacements: horizontally outward and vertically UPWARD. So, there is a chin shortening with it. You did NOT mention WHICH vertical direction of your proposed genio; up OR down. If a genio is to go BOTH horizontally outward and vertically downward a 'Down and Out' genio. Both the amount down and out can be LIMITED if the doctor does NOT sandwich a bone BUTTRESS (material to fill in the GAP it leaves) when the segment is totally separated from where it was cut.