My take on Kellaway/Donaldson in the No 15 jersey from Eddie Jones' point of view is that Jones sees Kellaway as consistent, reliable, and that this is as good a time to rest him for the following reasons. (But that he is the number one choice as fullback is a given).
With Donaldson, Jones is gambling that he will assimilate into the position with the other players around him in a game that the Wallabies should win, & win convincingly. Donaldson is really there for covering the No 10, both in this game & from on the bench in the future. He might well swap positions occasionally with Gordon from time to time during this game.
I would also suggest he will be fullback against Portugal, or even flyhalf to give Gordon a rest, which would probably mean Kellaway playing every game from here on .
Basically Donaldson has to serve as the reserve goal kicker. And no one else in the squad has experience at flyhalf ( which still baffles me)