I have been there! Very similar situations to what your in right now, more then once. I know it seems like there is no way out of the hole that your in but trust me, things will get better and it sounds like your doing everything possible to get her back on her feet again. Your not failing her.
(only knowing the nutshell version) I would talk to your vet about long term care of her back and ask if a chiropractor would help her at all. My horse is a klutz and slipped on the ice a few years ago and now every spring I have the chiropractor come out to just readjust his back because of how he walks on ice enow to protect himself from falling again it strains his back. I know lots of people that swear by a chiropractor. Maybe routine apts with a chiropractor will help her in the long term.
On the saddle thing, Im sorry you got ripped off, I know the feeling but if you now know she has back issues I would recommend getting a new saddle to avoid this happening again, sometimes a pad just isnt enough.
Keep us posted, im sorry your having a rough time. I hope I helped alittle. But remember, your not failing her and there is a light at the end.