I've used it, not an expert in any way. I've used up to 500 PPM concentration, I have seen no ill effects. I generally just guess, based on how seriously sick a bird is. I've put 1 - 2 tsp to a gallon of water, I've given it direct orally up to 1cc two or three times a day. Whether it works or not, I can't really attest too since it was not the only thing being done at the time, I'm usually using it in conjunction with other things treating and/or trying to boost the immune system. I've never had a bird turn blue (argyria), never had evidence of damage to liver or kidneys, I think you would have to do very large amounts for a sustained period of time for that to happen, but if a bird was already having liver or kidney issues that could be different. I gave it in conjunction with antibiotics attempting to treat salpiingitis, that treatment was not successful for that particular infection. Since your birds have been on antibiotics I would try putting them on probiotics also for a while, to help get the 'good' bacteria back in balance, and it may help with the overgrowth of the bad, but will take some time. Once things get out of whack, for whatever reason, it can take some time for things to get back in balance.
If the vet thinks this is bacterial vs viral (some virus's can weaken the immune system leaving them susceptible to other stuff), then I would look at the feed also, maybe an ingredient in your feed is giving your birds some difficulty with digestion which could contribute to the problem (it does happen that some birds don't do well on some feeds), or something environmental. Try to think out of the box and look around with an open mind.
http://blog.chickenwaterer.com/2015/03/probiotics-for-chickens.html