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I remember learning in physics (way back in the 80s) that air is the best insulator against heat flow. If the whole "greenhouse" thing is based on the insulating properties of a trace gas in the atmosphere--that CO2 actually raises the R value of air, rather than acts to hold heat--then I'm less convinced than ever. An insulator slows heat flow; CO2 would act equally to slow the flow of heat in from the sun as well as out. Venus is not a good example of the "greenhouse" properties of CO2. Being significantly closer to the sun than the Earth, it receives significantly more solar energy. Mercury is even hotter, and I don't think it even has an atmosphere.

Water holds heat a lot better than air. That's what I've always considered responsible for the ability of water to act as a greenhouse gas.

I don't recall ever reading anywhere that the earth was hundreds of degrees hotter when the CO2 content was thousands fold higher than now.

Air is only a good insulator when it is trapped in pockets or layers. I'm no physicist but I believe it is because air only conducts heat by convection (movement of the energy laden molecules in the fluid). This is why clothing (wool, and other materials that insulate, like dual panes of glass) have air gaps/pockets/layers in them.
Air by itself can and will transfer energy. It will try to obtain an equilibrium, and use forces like convection to transfer energy around.
That is why air by itself (like the atmosphere) unrestricted is not a good "insulator". Air trapped in small enough pockets must try to convect energy, and then radiate it through the outlying material. That is inefficient transfer of energy.

The greenhouse effect is something slightly different.
Think of a green house. You are trapping air inside, and you are allowing energy in (ultraviolet and infrared radiation) and you trap or insulate it with a material so that the air does not deliver the energy back out through airflow. Then, you can grow plants in your greenhouse when the outdoor temperatures are lower. You allow energy in, but you slow it's dissipation.

The atmosphere has some greenhouse effect. Water vapor or humidity is a good relative insulator. This is why arid areas (deserts) can be so hot in the day and get so chilly at night. The heat is allowed to dissipate because there isn't as much atmospheric humidity to hold it, and to trap it.

That is why I say we should stay focused. There's no reason to debate the greenhouse effect itself, really. You can try, but I think it's a diversion.
The issue is CO2.
CO2's greenhouse effect is relatively negligible compared to water vapor, particularly at the levels it exists in earth's atmosphere, but there are always other things to consider, which is why the science is somewhat theoretical. If you increase some gasses levels in the atmosphere, it changes other properties, not just greenhouse effect. For example, some gases may help insulate or hold heat, but can also increase the reflection of solar energy.