Unfortunately, a lot of people are finding that DE is not an effective treatment for external parasites.
Check your local feed store, they should carry Permethrin based poultry spray or dust - these are effective and safe to use when following the label http://www.backyardchickens.com/t/1151513/pesticides-approved-for-poultry
We all love to give our girls (and boys) treats, but it's best to limit the amount of treats to no more than 5-10% of daily intake. I do truly believe that some of the larger dual purpose breeds like Orpingtons, W Plymouth Rocks, etc., may be genetically predisposed to being a bit overweight or it could be that they are just not quite as active as other breeds.
I hope all goes well for you and that your sweet girl continues to improve.
A good read here on what can happen to overweight hens: http://www.the-chicken-chick.com/chickens-obesity-silent-killer-how-to/
