This is one half of a bulkhead. Below it are the vents for the aft hold, as well as (at least when I was there) a 20' flatbed for stowing container lashings.

Well,, my thinking would be that underneath could be a storage for lifting equipment/wires etc etc, but honestly it,s a guess from my side

Why those tweendeck pontoons can't be placed on deck or right on those supports the "plate" stands on? And then the space underneath it looks unused.

Denis,could be a stowage plattform for the tweendeck pontoons ?

What could be that "thick grey plate" in front of the superstructure? Doesn't look like hatchcover(s) or bulkhead(s) & it's too thin for a stabilizing pontoon.
Nevertheless, this is nice warm photo!