Yeah, not saying definitively that broad mite are what have been causing you grief, but from what you have said, it does sound plausible to me (based on my own experiences).
Firstly, as it sounds like you are already aware, white fly are a major vector of broad mite--the little mite bastages love nothing better than to hitch a ride on 'em--and it sounds like your plants are no stranger to white fly attack.
And secondly, from what I myself have personally observed, it really does appear that broad mite *do* favour certain plants (and I do mean plants; not genotype/variety or species). Usually they start out on what appears to be a few random plants here and there but eventually spread as their numbers increase. When I have been successful in keeping them under some control, re-infestation pretty much always occurs first on the initial plants they started attacking!
Under a magnifying glass, broad mite should at least look like tiny spots of white dust if that is of any help. If you cannot see even that, you are probably correct in the assumption that the said treatments against white fly have also knocked out the mites. Well, of course, that's assuming too that broad mite are indeed the culprits.....
