Don't forget about calcium, very abundant in seawater. In fact, is not seawater saturated with respect to calcium carbonate -otherwise the sea shells would all re-dissolve?

Also, the carbonate/bicarbonate system is not the only buffer system in operation in the oceans, borate is also significant in this regard.

<<A solution of NaHCO3 or Na2CO3 is equivalent to adding a stoichiometric amount of NaOH to a pure solution of CO2>>

I'm not sure about this statement, well all right just the NaHCO3 part of it. Surely bicarbonate is fully saturated, it cannot take on additional CO2 equivalents?