You don't need to capture anywhere near the total amount emitted into in the atmosphere. Just enough to offset warming.
If you start reversing acidification, the ocean is just going to start pulling in more CO2 from the atmosphere.
1) Nobody really knows how much is that. 350ppm? 300ppm? 250ppm?
2) Don't forget about feedbacks which have already been triggered and will keep adding warming regardless of what humans do.