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.
Plastic in the oceans is not a worrying amount of carbon, it's just a worrying amount of plastic.
Even if you burned every single kilo of plastic produced, that's still "only" 380 million tons a year, or about 4 days worth of CO2.