Selena's back (from just behind the scapula to her last rib/supportive vertabra is just about 16 inches....welcome to my nightmare
I did however, try a small 16" wintec dressage saddle today and it seemed to fit both of us pretty well. I attached a picture below.
I think I will continue to experiment with a few others until I have a better idea of what will work for us. We plan to do a 15 mile CDR this weekend in Woodstock, VT and will be using the treeless again, so we shall see how that goes. Thanks again for all your help!