Can't see how defence would be an issue for him at fullback, any more than it would in his current role, or at hooker for that matter. He has the speed to cover, and technique he can learn.
As for catching, fuck, Barba actively ran AWAY from the ball when he started and went on to win the Dally M....so again, this is a skill that he can be developed.
He's got the tools to make a go of it. Recently his cleaning up of grubbers in goal, and subsequent runs out have shown promise....why the fuck not at least experiment and find out over the next month or so?