I am presently riding a horse that moves well and has what I think could be called a ‘weak back’. I have had him over a year. Started out with uneven muscle and what can only be described as a ‘hole’ behind his right wither. We thought we had this behind us and had a very good year showing last year. In January he dropped just enough weight that by the time we noticed it saddle fit became and issue, blanket pressure added to it and we are now just getting back to where we were the first of the year. It can be very discouraging.
So. My trainer really likes this horse and he is very sensible which makes it almost worthwhile. If your trainer is against it you might be paddling upstream. Is there any way to get the mare for a month(or longer) trial to see if getting her in shape helps her back? It might be worth it to the current owner to get the mare is shape and add to her training ( make her more sellable).
There is no telling what will be the outcome for either horse or that the next one you see won’t be better then either of these two. I know that this thought does not help. But there is always another horse. Good luck and let us know how it turns out.