I actually just brought her to the vet yesterday to get her teeth floated and they didn't mention anything about her back. But I've also heard some vets don't want to offend horse owners. I will ask next time for sure
Sway backs actually don't hurt horses... It shouldn't affect her. I've seen horses way worse then that. She should be fine (: If you want, just go ahead and have your vet look at her, but I know 2 horses that have worse then that and they are active therapy horses.
There is a saddle pad made especially for sway back horses by Reinsman. It actually has padding that "bridges" the space between the withers and the rump and makes the saddle fit really good. It has the "tacky too" backing so doesn't slip.
My husband used one on his old horse on the trails, hunting and packing and he had a sway back as bad if not worse than this horse.