That crazy, and im very surprised they survived. Optimum temps outdoor would be 26-29 day and 22-24 night, about 50%rh.
Next year use a lot of mulch, 1/2 shade and water daily.
No it doesn't work. All your doing is killing the existing mites , not the eggs or the cycle. Miticide , regular vacuuming and if you've been in the outside garden don't go in there until you change shoes and clothes and shower.
Bury a barrel/container with large lid underground, vac seal(not too tight) to fit more in. Its cool, dark and secure . Don't let anyone see you going to and from it. Try to put in sandy soil(easier) on a slope or up high to avoid flooding.
No you need H202(30%+ grade) in there , add it to the water every week. Looks like a diseased root system issue and the others will end up the same if nothing done.