Pumping oil from inside the Earth and then converting it into plastic and then into CO2 has a much bigger impact than putting the plastic into a garbage dump. The oil to make the plastic came from a hole in the ground. A garbage dump is a hole in the ground. You've essentially done nothing. If you decompose the plastic into CO2, then that has a major impact on the atmosphere.
This is a good point I hadn't thought of. One adjustment to your calculations is that the CO2 emitted from plastic would weigh over 2 times the weight of the plastic due to the additional oxygen atoms, but that doesn't change the calculus substantially.
Absolutely, but if the choice is "burn" the plastic if you could (via an oxidative process like digestion, rather than combustion) in place in the oceans or just leave it there forever, then adding to the carbon in the atmosphere is not the thing to worry about.
Oil deep underground somewhere in the middle of nowhere is not remotely close to landfills close to surface, next to understand waters and relatively close to cities
It is more efficient to burn the already refined petroleum product
(plastic) than to dig up more unrefined petroleum products (crude) and put them to use
