The question isn't whether CO2 is a GHG - as you say, we know it is.
The question is, "Do increases in CO2 have the ability be the primary driver in altering global average temperature". In other words how effective is CO2 as a GHG.
The answer to this was provided by doing a simple correlation between temperature and CO2 on ice core samples. With higher amounts of CO2, temperature increased. The only problem is, once people started to look at the timing, it was found (as you point out) that increased CO2 was not the cause of the interglacial periods.
So we can agree that CO2 was not the cause of historic warming. Then why does the IPCC and the climate change messiah, Al Gore, continue to trot out graphs of CO2 and temperature over the past 600,000 years and point out that high CO2 level coincide with the warm periods (interglacials)? Could it have anything to do with the fact that, without that graph, it might be tougher to sell dvd's, or continue to get government funding?
CO2 was believed to be a very effective GHG based on correlations in historical evidence (for which CO2 has now been shown to be the effect, not the cause). If you want to change gears to saying CO2 is a very effective GHG based solely on physics/chemistry, that's fine, but just be up front that this statement is based on theoretical climate science, of a system we do not even come close to fully understanding.
