It may be coachable to make him a RB but in reality he doesn't have the raw assets of a FB and is that what we really are a club that just coaches specific players for ages... For me that's the cheap option. Just buy a quality RB and let Japhet plays where he plays best.Every thing you say here to my mind us fair. But more worrying the issues you raised are clear and overtly coach able. I makes me ask for the umpteenth time what do they do all week.
Kind of reminds me of Danny Rose saying he was only ever taught the positional requirements of Left back by Pochettino.
Japhet has potential he also is a hard worker with heart. Send him on a season long loan to championship team that will play him at CB. He has potential to tap we can't waste anymore of his development time.
100% loan him out somewhere next season where he can play CB preferabbly English team.