Note that Mitsui have a 25% interest in the 3.9MT of rock...

Note that Mitsui have a 25% interest in the 3.9MT of rock phosphate. That isn't a particularly big slice of the world market.
You have suggested that RWD would be too small for Mitsui, I am pointing out that they have taken much smaller relative market positions in other agricultural nutrients.
Your comments on future SOP margins are pure speculation but a legitimate concern. If the margins fall they will fall to the level that sees the Mannheim producers go out of business. That would be the price of MOP plus sulphuric acid plus production costs.