At the end of the day the Captains decision to place the least qualified officer on the flight deck "in command" was the critical hole in the cheese (in combination with the CRM deficiencies that precluded the PM from assuming control).
It doesn't matter a damn who the guy nominated as his replacement i/c.
With an aircraft approaching the ITCZ, and with two F/Os of limited nouse, there is no way that captain should have left the flight deck.
Many times have I been in a similar position. I would say, "Oh, I'm not tired, I'll chill out here....", or some such.
I would only ever leave the flight deck if there was at least one 'hairy old SFO' there, and even then ONLY if there were ZERO tech or wx problems.
This captain must have been barking mad to have left the flight deck for his 'rest' whilst the wx radar was a mass of red and purple.