The people who advised adding salt, did they say what the salt will do? It isn't magical. Salt has some anti bacterial, anti fungal and anti protozoa properties it will also help ease some osmotic problems and that's it. Because this condition has gone on for this length of time I think it is safe to say that it isn't anything to do with digestion, blockages ect because if it was the fish would be dead by now. This leaves a true swim bladder problem and that is extremely difficult to cure. In a minority of cases an infection is the cause but when it is internal the usual antibacterial remedies are generally ineffective but an injection given by a vet would be effective IF this is caused by an infection. If the cause is physiological then quite simply there is really nothing that can be done other than surgery!!! Add salt if you feel it will help but all you are likely to achieve is slightly lowering the O2 level.
People love to give advice. People who have some basis for their advice won't mind explaining what they expect the salt to correct, which means they also can tell you all the details. But in most cases they're going to say "I heard it somewhere" or "it helps them" or other silly things that lets you know they have no idea. If they do have a theory and can explain it then at least you have some basis to make an informed decision.
I hope the people telling you to add salt warned you of the risks, explained the amount to add and for how long. Also hope you have a salt test kit, and have a way to dispose of the salty water safely, and don't have a problem doing the large water changes needed to reduce salt levels when the treatment is over.
There's no huge problem adding salt if done correctly. Fish can normally live thru it.
If you're adding salt only because someone said to, no additional info, you might consider what path that puts you on.
Interestingly the buoyancy issue could go away during the treatment...on it's own. But the salt will get the credit.
Is she doing it anymore now that the weather is colder? It could be a slight swim bladder problem, which can be fairly common in older fish. You will need to keep an eye on her during the winter in case the pond freezes over. My suggestion would be to look at covering the pond to keep the chill of it.