you need to cut away ALL of that dead plant matter. that's only making the problem worse. after that i would vacuum the plants and then give them a really good rinse with mighty wash. then spray the topsoil and the lower stems with neem oil. follow the directions on the bottle. keep checking on it over the next week or so. check under the leaves and on the buds. if you still have problems then i would hit the buds with neem oil as a last resort. i think your problem is that the mighty wash is getting rid of the mites on the plants but not getting them out of the room and they are just coming back in a few days. washing them and then getting neem oil on the topsoil and lower branches might help stop that.
after this harvest you should get some doktor doom total release fogger and bomb that room with all the gear (except bulbs) in the room. then get in there and clean everything...the lights, hoods, fans, walls, etc. that and neem oil on the plants like i described before whould help a lot.
also, with neem oil, gotta give your plants a couple hours after spraying before you put them back under the light. if you run 18/6 veg then just do it right before dark cycle.