It isn't too late to treat with oxalic - the current cold spell will leave 99% of the varroa mites on the bees rather than in the cells. The problem you describe on the mildew is through ventilation - I presume you have a national hive ? if so, try putting some twigs/matches around the inside of the top board/crown board to 'jack' up the roof once fitted - and also ensure that at least one feed hole is open.
If your hive entrance isn't south facing, now is the time to get it as close to that as possible - the winter sun then encourages the bees to clear out any waste etc.
The final thing you can do is to ever so lightly tip the entire hive forwards, so it is just off level for the winter - this helps any water in the hive (condensation from 1000's of bees breathing!) to run out via gravity.
I have noticed that on my hives, those with an open mesh floor do tend not to have as mush of an issue with mildew compared with those with a solid floor, although I used to find my old WBC never suffered from it - the huge double wall acting as a great insulator !
regards and welcome