How about some good ole biochemical miticide sucrocide, three treatments, spread 7-10 days apart?.............just to knock down the mite load and get it back under control. I would do this rather than take a chance of loosing the hives. Once they've bounced back, I'd rethink the apiguard application and make sure I applied it correct.
oh yeah...........EPA approved for use in bee hives
Good-luck!