I have found that the trick is to really clean the damaged area very, very well and aggressively. A soft bristle brush doesn't do it. I have used a brass bristle brush to remove the dead material before every treatment. Only after that did i see great results.Just a thought for you...
I have found that betadine is best used once to try to really kill the infection after you have cleaned out any active rot best as possible. Any subsequent treatments of betadine actually will also damage the good tissue and retards healing. I get best results with an initial treatment of betadine, then keeping a good anti-fungal on after that. I like vet quality silver sulfadiazine cream, but have also had good luck with plain old athlete's foot cream.
Love the turtles and really appreciate your success and dedication to these turtles. Thanks for sharing.