Re: "treatment treadmill"
Vance, One serious question I woudl have. since you evidently moved and handled all 30 hives. do you think maybe you carried the Foul Brood to all 30? Just asking. You could have had only one infected hive and then been the carrier to every other hive. You did say they developed it. Not that they already had it and that is what stood out to me.
I don't tend to hold another beekeeper responsible for what hives develop under the care of another beekeeper. Not that it couldn't be so. But the first place to look for me is the current beekeeper. That is the first and most obvious answer.
You also said the beekeeper sold out due to not being able to continue. I am sure he did not just wake up one day realizing he didn't have what it took. It is reasonable to assume that his bees have been in a state of decline as to the quality of their care for some time. I expect that if the bees came with a pre existing condition it might be due to lack of care rather than care masking symptoms. The timing of it appearing may have been coincidental. But I suspect it a result of lack of care far more than I believe it is a masking due to care. The odds are simply in favor of it given the other factors.
What you do relate is in fact. The bees where doing fine under his care including any treatments he applied. they did not do well under yours at first. you then treated in your preferred manner and hopefully they will continue to do well. Both remedies are effective. you simply did not keep up any remedy at first. Evidently they where well enough when you chose to take them to pass your evaluation.
Treated bees are well, untreated bees did not do well. bees again treated but with different method doing well again. Seems to me treatment is necessary and their is a range of choices.
Significant Minority vs Insignificant Majority, 20% of causes produce 80% of the effects.