I agree with your Niger Delta comment, but not with your 2nd point. It really comes down to defining what "worst" means and I'm not sure is that simple to compare all those completely unique disasters (and it's even harder to put the pacific gyre - I assume you're talking about the garbage patch - on the same category as bhopal/chernobyl) specially when you consider the probable long term effects they might have.

Regarding your 1st point, I think the different shades of black on the map reflect the density. Even though we still don't know what a 'light grey' or 'black' looks like in practice.