asked, "Any chemists here who can tell me exactly what “chemical alteration” would “Suck COS out of the atmosphere?”"
I would say the lime cycle. IE: heat and water.
There is more CO2 on earth then there is on Venus. Which I believe if we pull a large comet(water) to Venus we could have a livable planet in a few thousand years.
photosynthesis would be slower but will fine tune what is needed for us to breath.