Is there any way you could post pictures of these scales? The rust coloring sounds like belly rot. Are the cages really damp/wet? Just as a preventative measure, take them off of whatever substrate they are on, and replace it with clean, dry paper towels or newspaper. Keep them dry, and apply neosporin on the affected scales 2x/day or more if you can. If you have silvadene you can use that also.

It just seems like there was alot done to get rid of those mites and it may have irritated their skin more than necessary. The Provent-A-Mite will work by itself, nothing else should have been needed. The betadine just seems to be a drying agent. It cleans well, but I don't see how it would be a good preventative against mites. Just wondering, is it Betadine solution or betadine scrub? The scrub has soap in it and really needs to be washed off, even more so than the solution.

You can take the snakes to a vet and get a skin scraping done to determine if it really is a fungal infection or not. Not really diagnosable at home.

Do any of the snakes still feel oily? If so, they may have had a reaction to the oil, causing the scales to look the way they do. Can't be for certain, but I would just take them to the vet just to be sure.

I hope this has helped a bit, and if you need any other suggestions, let us know.