'He already plays like a 5/8 but without having to tackle' - seems to be a popular opinion here. I'd rather move him because he is useless positionally and usually shows no urgency to retrieve kicks.
We are often on the back foot early in sets because Hayne cruises and our first tackle is about 10-15m behind what a normal fullback would get the ball to. He can be dangerous bringing the ball back but how often is that? most of the time he jogs back, slowly picks up the ball, jogs to the defensive line and meekly surrenders. He will only bring it back hard or fast if he spots a gap.
Since he started to really play like a 5/8 in attack he doesn't back up as much as he used to, fullbacks should always be running alongside forwards, or anyone running with the ball, he rarely does that now. Look at Slater, he does it all the time and it leads to a lot of Melbourne's tries.
So IMO if he is going to play like a 5/8 he should have the 6 jersey on with a fullback in the team that is going to play as fullback. Hayne shouldn't be back there just so he doesn't have to tackle, then step up when we have the ball like a quarterback. It's effectively weakening the 1 and 6 just to accommodate him.
The only problem is though, if he was moved to 5/8 he'd spit the dummy again and just go through the motions like last time to force the coach to move him back to fullback.