the powdered sugar does the same. the grooming idea is legit, i think, but more depends on the bees and how good they are at managing the mites. a knockdown of mites can give the bees the edge if they are good at their own mite control. if they are not, it's not enough. in that case, you have to decide on treatment, or letting the hive go.
i think i have almost gotten to treatment free, but i have been fortunate to pick up some swarms from some true feral hives, and use that stock for queens. even so, along the way, i have lost my share of hives. not always to mites, but in a couple, i'm sure the mites played a role.
in one of those hives this year, i found a really high mite count. i was sure i would have to treat them. i dusted them with PS twice in a month. now the mite count is really low. this hive is going into it's 3rd winter with no treatment. we'll see what happens.