What is also interesting is that the pilots discussed the wx ahead. They told the cabin crew that here would be an increase in turbulence. How did they know that? Were they planning to fly through some of the returns? Flying through echo's at level 350 is a bit iffy, not something I would do voluntarily and not something I can imagine the captain leaving to the two FO's.
So here is the retrospective catch-22. The wx could not have been that bad, otherwhise the captain would linger a bit longer in the flightdeck. However the wx was bad enough for the FO's to warn the cabin crew. Was it that bad that there was no space to properly avoid the echo's? In that case, why was the captain away? The PNF suggested tentatively a turn, but only a turn of 12 degrees was made. What were they avoiding?
I have never flown across the ITCZ, so maybe others can comment, but again I feel as if we are missing a lot of information.