Queen disappeared, winter approaching!

I did a split a couple of months ago and introduced a mated queen purchased from a professional beekeeper to the queenless split. The queen was definitely laying for a period of time but on a recent inspection I can't see any eggs, and no queen. The worker numbers have dropped, as has capped honey. I noticed about 4-5 opened queen cells on the existing frames so the workers certainly tried to make a new queen.

There aren't any queens available due to the season. Can anyone recommend the best course from here? I've fed them a good supply of fondant to keep them happy but I fear unless a mated queen returns to the hive soon the colony won't last long.