If, as you say must be the case, CO2 settled on the ground displacing all other gases, including oxygen, then we would all be dead! It seems to me the atmosphere is constantly mixed by the winds and convection currents within the atmosphere.
I know you are an engineer and they are all good questions but why don't you do a few Google searches and answer them yourself?
I think you'll find the many thousands of highly credentialled climate and atmospheric scientists have spent millions of man-hours researching these questions and more, and with the fast search capabilities of Google we can find the answers extremely quickly and do not have to waste time coming up with smart sounding but ultimately baseless objections to climate science which have already been answered many times over.
I don't have time to do it for you I'm afraid. But your first assertion defies common sense and is therefore plain wrong so I suspect the rest of your points, while interesting, are going to be of a similar quality.
P.S. CO2 is not the only greenhouse gas. Water vapor, CO2, CH4, N2O, and SO2 are all greenhouse gases and, yes, the most important is water vapour.
P.P.S. Why should the government take any responsibility for addressing climate change? It is for the greater good. Taxation is also for the greater good. Therefore, why should governments have any involvement with taxation?
