If it's a Varroa-free area are there other beekeepers you could contact? As Lindsay says, the importation of mites is effectively irreversible. There are so few areas left that are Varroa-free that I think we have a responsibility to protect them if we possibly can. I'm sure the house owner is keen to have bees on the property but perhaps you can persuade him/her that a little patience now will allow the keeping of bees without chemicals.

Personally I wouldn't trust any combination of treatments to make the hive mite-free.

I've got colonies that gave a drop of ~25 mites over 6 weeks with Apivar early last autumn. These colonies have had fantastically low counts all last year. In midwinter they got oxalic acid/Api Bioxal ... and one or two more mites appeared.

Perhaps you could persuade the house owner that, in the absence of mites, they could also get to see the occasional Braula