I had all this in mind, but was too lazy to write it all out, so thank you.

I agree that Le Ch implies that extra H+ pushes (4) to the left, hence (5) to the left also, assuming nothing else has changed.

But extra CO2 dissolved pushes (3) to the right, thus increasing the quantity of HCO3-- by exactly the same amount as you increase the H+, (unless there is something else not mentioned here that consumes either). It seems to me that if you increase both H+ and HCO3- in the same proportions, then (4) on balance doesn't move at all, nor (5). Though I can possibly conceive that there is something else happening outside this system which means that at the starting point H+ is at a lower concentration than HCO3- (other components in seawater are keeping the pH to 8 remember); then if the number of H+ is increased by the about the same number as HCO3-, then that is actually a larger proportionate increase in the concentration of H+ than the increase in concentration of HCO3-, so that would push (4) to the left.

But even then it still seems counter-intuitive. Carbonate-producing life forms can't produce any carbonate at all if there is no dissolved carbonate in the water. So the first addition of CO2 from zero promotes carbonate formation (assuming for simplicity that there is no other source of inorganic carbon, aside from the carbonate so deposited). Then at some concentration it reverses, and now more CO2 now reduces carbonate formation, that is what we are being asked to believe. Now there is nothing in those above equations that, as far as I can see, indicates a change in direction at any point as CO2 is increased. So I think there has to be more to it than just the equilibrium chemistry. That is why the argument about super-saturation of CO3-- being possible at higher pH, rather than equilibrium chemistry, appealed to me. Obviously there would have to be a source of supersaturated carbonate coming in from somewhere. There's probably plenty washing off the land.