I would like to know how many of those feral collonies were escapies from large cell beekeepers, and how many were not fully regressed. From my understanding bee taken from large cell will build their first natural comb at 5.1 or larger. Then when those newly hatched bee are taken out to start over, they will build smaller, but possible still not all as little as 4.9. They might need another regression period.
It is after they are stablized, totally off chemicals, oils, and such, and fed natural healthy diets, that they then start taking care of the mites themselves.
After the mite infestation, were the survivors all small cells ferals?
You have 100 hives. You wake up one spring morning and they are all dead. The first 20% you check you determine died of mites. Do you then just assume that all of them died because of mites? Or do you look further.