>who is controlling...
Depends on which port and which personnel.
The better informed are all aware that after discharge, if the remaining barrels of benzene or methanol can be completely removed by suction and evaporation from the designated edible oil compartments and all associated pipelines and hoses, there is never any cleaner equipment.
Considering earlier voyages, there is no way 12 hours of shipyard cleaning is going to be as effective at removing traces of previous cargoes like acrylonitrile or spent transformer oil, compared to a full voyage when loaded with hundreds of tonnes of such a clean, strong solvent as high-purity benzene or methanol.
So you can only imagine whether everyone involved will all play their supposed part to the fullest extent you might have in mind when preparing for bulk edible oils. Every time.
Sometimes expediting the movement will have to take precedence, other times there "just might" be the attitude of "why bother?"
And when you're the one climbing down inside the vessel compartment to inspect for cleanliness, there can be quite a bit of pressure not to reject a tank that looks so clean (as long as there is no obvious wetness) compared to some of the other choices on board the same ship. Once any of the crew or yourself has been down there and come back up without immediate ill effects, it's been long thought it can't be that bad unless you actually saw something wrong visually.
Certificates of Cleanliness, if required, will always be a part of the paperwork package.
And may I remind you it's the methanol that carries the skull & crossbones, not the benzene.