I agree with your points. Nicely laid out btw.
But people like the OP, with legit credentials, say otherwise. If they are right there must be a forcing that is overwhelming the natural ocean buffer system. Have we run out of calcium or other metal molecules that bond with excess CO2? I don’t see how that could be possible.
Neither can I think of another reason that an ocean’s pH buffer system would be inactived by excess CO2.