I duobt Wick provides all the drivers for the route. There are actually four out and back trips each way. Two of the four* units needed to run the service start the day at Inverness. And since the units take the best part of twelve hours to do the round trip, including layover times, I'm guessing drivers don't go all the way there and back but swap over en route.
We do know, from the accounts of the stops on 1/3/24, that the driver of the first northbound train (the second to be stopped) was not driving that unit when it came back from Wick three hours later (to be stopped as the third train), as Appleby refers to these two drivers by different personal pronouns.
* actually six, as the units that start the day at Wick are not the same as those that end the day there.