If we have a hive reduce to 2 or 3 frames then we would put them into a 5 frame nuc with honey or reduce to a single with honey and pack out the sides with polystyrene to reduce the volume the need to keep warm.
No experience of varroa, but the reduction in numbers can happen without Varroa, sometimes in mid winter it is due to the bees not breeding in Autumn and so the geriatrics die off over winter. Autumn breeding is very important to us to have bees available for almonds in early spring.