Ammonia poisoning can cause several secondary problems such as bacterial infections and parasitic infections. Overall, I'd say AQ salt is still the best choice because I have no way of telling if he has a bacterial problem (it would be internal). He isn't bloating or looking like he's retaining fluid, which are usual symptoms of internal bacterial problems. I can totally understand your frustration. It's awful not knowing how to help your buddy (been there so I know). But treating him with meds when we don't know what he has could just make him even worse. That's the heartbreaking problem with fish illness: so often we don't know how to treat them for lack of symptoms. :(
If he's in a 10gal, you don't need to do 100% water changes every day. You can do 50% once or twice a week. Just keep careful track of how much you take out so you know how much to put back in.
The silver could be from ammonia burns. The opening of his mouth sounds an awful lot like the "yawning" that many fish do so it could be a natural thing and not related to his illness.
His fins most likely have a bit of rot and ammonia burn and that's why they're gone. Do you see any new growth at all? Is there any black edging?
Also, the AQ salt can only be used for 14 days and I've lost track what day we're on now. :/