14c is not at all too cold for a fukien in winter. I keep mine at night temperature 7c. On sunny days it can go up to 12c. I think moving the tree to the extension would be a lot better for the tree than the current position. It will also significant decrease white flies reproduction speed.
Several years ago I had a major problem with white flies and could not get rid of them. Somebody on the forum here suggested getting a product whose name I can't remember. It is yellow cards with very sticky sides, hung from a branch, which attracts white flies.
I hung one up and in a couple of days it was covered. I hung another up and it caught a lot more over the rest of the winter. Next year there were some. Last year and this year, none.
I just found a similar thing in Amazon by searching "yellow sticky insect trap". It involves no poison or harsh chemicals on the trees.