Your question assumes that the oceans had to have been acidic because CO2 was high. This assumption is wrong.
You don't get acidification as long as CO2 levels change slowly enough that it mixes down to the bottom of the ocean and then gets buffered by calcium bicarbonate being dissolved there and mixing back to the top. But this mixing takes place on the scale of a thousand years. This is no big deal for CO2 level changes taking place over geological time. But it doesn't help shellfish with sudden increases of CO2 taking place on a scale of decades or centuries.
Another geo-engineering idea is to speed up this mixing by pumping air deep down into the oceans. Personally (as a PhD geoscientist), I think that taking CO2 out of the air, compressing it to a liquid, and re-injecting it back into the geologic formations that the oil came from in the first place, is the procedure for getting rid of CO2 that is least likely to screw things up even more. It would just go back where it came from (many caveats, but basically can work).
Start raising a tax on CO2 emissions and at the same time start paying people to put it in the ground. Raise the price on both each year and you could reach equilibrium pretty fast. Should have started 20 years ago, but now is better than never. Maybe start at increasing it at $10 dollars a ton/year. $10/ton is about $4 per barrel of oil. If people saw that sequestering CO2 would pay $100 a ton in a decade the research and excitement around the ideas would be huge and likely encourage many breakthroughs. Isolated solar and wind farms extracting CO2 from the air and pumping it underground could be a huge business.
You can say that if you like, but I tend to err on the side of scientific evidence. Even the hysterians most scary claims merely suggest that we "are headed for" ocean PH levels that are "unheard of in 25 million years". And yet the shellfish people are trying to tell us will die from slightly thinner shells have been around for a couple of hundred million years.
You don't get acidification as long as CO2 levels change slowly enough that it mixes down to the bottom of the ocean and then gets buffered by calcium bicarbonate being dissolved there and mixing back to the top. But this mixing takes place on the scale of a thousand years. This is no big deal for CO2 level changes taking place over geological time. But it doesn't help shellfish with sudden increases of CO2 taking place on a scale of decades or centuries.