I figured I'd take a stab at it. I can see the complications as well and I've had at least one client like this. He had very high shoulder, erect posture, yeah a very forward and sunken neck. The shoulders of every jacket he owned stood up a solid 1.5" off his shoulder and the collar stood at least 2" away from his neck. When I put him in MTM, I had to use "stoop-ed" in the posture description even though I knew it wasn't quite right. I had to make adjustments on the final product but it was way closer than anything OTR he ever had and he was ultimately happy. Glad I put it out there that in this case my lack of experience wouldn't make my diagnosis entirely certain.
Would you not say there is swayback posture, as well? If so, that doesn't necessarily affect the upper portion of the body in the shoulders/neck so much, does it?
By the way, I
Would describe your posture as erect, head forward, half high to high shoulders (can't tell specifically from these pictures) Forward hip/flat seat. If you say stooped you will get a longer back than front so don't say you are stooped. This explains the disparity of the jacket fit.
This combination is one of the trickiest to cut for because each adjustment counters another. You need a shorter back/ longer front balance to compensate for an erect posture but you have to add length at the top of the back for a head forward and at the same time square the shoulder to match your shoulder line. You have to square the shoulder by adding cloth at the shoulder point because you take away necessary length if you square down at the neck. This all has to be done by cutting because it is almost impossible to adjust a finished garment. Very complicated fitting issues here with no easy solution.
Thanks for the consulting, guys. I'll look for a suit with the least-bad fit and stick to more casual clothes whenever I can get away with it. Since MTM won't work, I will put those pennies toward a bespoke a few years down the road.