This is one of those areas where I don't think we can evaluate desirability of a colony or queen strictly by resistance to mite load. These hives are not closed systems but rather dynamic and changeful. It may make us all a little crazy to believe a job is done when it's not. A course of action cannot be approached in a linear way when so many outside variables exist. That's what makes beekeeping an intuitive art as well as science.
Really happy to see you not give up on this colony.