EnduranceLover6, congrats on your recent ride especially the 99% vetting!
I too have a very short-backed mutton withered horse that I have had many saddle fitting difficulty with! He is a Rocky Mountain horse 14.3h and wears a 69" sheet! I am not sure about the rib lengh measurement you are refering too, but she can't be much shorter than my boy!
I currently am riding in a Fabtron saddle. It is a 15" saddle and the only one I have found to fit. It was the saddle I used for kids, and it has fit every single horse I have tried it on! Including a 16h AngloArab/Percheron with a wide, long back; a 14.1h QH mare; a 15.3h skinny TB/Paint; etc.
I had never used it before, thinking it was just for kids, but one day sat in it and realized it was an adult saddle...so it works!
I happen to have two of them, same size, make & model and have one of them for sale. I would sell it for $200 + shipping OBO.
I am planning on trying the Barefoot Tahoe on a Demo basis cause I hate the horn (I may just saw it off one day) and I read the Barefoot all have the support system you were refering to. They have models as low as $600 new, so you might want to look into them as well.
I am posting a picture of Chivas my RMHA gelding in the Fabtron and the Barefoot Tahoe I am going to Demo.
I would never sell both of the Fabtrons, because they seem to fit any horse, but I don't need two of them anymore since my girls both ride in 16" Wintecs now, one Pro Dressage, one Pro 2000.