Re: Still too many mites
I live about 200km south of you (Seattle area) but our climates are not so different. I think it is now a little too cold for Formic and as you said, you cannot do formic while feeding.
Was it several dozen drop per 3 weeks? if so, I would leave it until I start a series of OAV in December (I usually do weekly OAV until mites stop dropping). However, if it was several dozen per day, I would immediately put apivar strips in the middle of the broodnest. Then, before it gets too cold to open the hive, I would move the strips to the top box (hopefully the cluster will have moved up by then), so that the strips can be removed easily at the end of the 6-week treatment period. Or, you can immediately start weekly (or more often if you can) OAV until mites stop dropping.
Got 6 gallons in October and needing more? Maybe you do not have much English ivy around or your bees are super-prolific. But you may want to put a robber screen.
Zone 8, elevation 70 ft, near the north end of Lake Washington