I wouldn't move Stewart from fullback. Centre is a completely different scenario, particularly in defence. Fullback is also a position that handles the ball a lot more than Centre. Why weaken fullback to strengthen the Centres.
I'd imagine Johnson is most likely to get the gig as this will have the least disruptive effect on the side.
Of course, we could all wait till the judiciary reports come out this arvo before we jump to hypotheticals. But given that we seem to have antagonised the NRL over the last 24 hours, I can't see us getting any favours.