You are not explaining the reason why CO2 is more opaque to IR radiation.
I suspect that the reason for this effect is simply that the CO2 gas has a higher density.
As a comparison; air that contains water vapour has a higher density per cubic meter than air that has no water vapour, therefore the air that has water vapour can retain heat for a longer period of time.

It is true that there are various indications that the Earth's climate is changing, and there is indication that the CO2 concentration in the atmosphere is increasing
Such as on the following web site:
http://en.wikipedia.org/wiki/File:Mauna_Loa_Carbon_Dioxide_Apr2013.svg

Certainly there are many sources that generate CO2 and introduce it to the atmosphere.
But is it not also true, that IF the concentration of the other gasses is reduced, then the CO2 concentration per cubic volume would seem to increase?

We know that CO2 gas is heavier than air, even though it mixes with the air, being heavier, it gravitates closer to the ground.
Suppose the following scenario is true: Heavier gasses such as CO2 and CO, etc., have a greater concentration close to the ground, and more of the lighter gasses such as nitrogen and oxygen exist very high in the atmosphere.
Now suppose that the gasses in the upper atmosphere disappear into space, would that not leave a larger percentage of heavier gasses in the atmosphere?
The gasses CAN escape the Earth's gravitational field if they have enough kinetic energy.
See the following web site:
http://science1.nasa.gov/science-news/science-at-nasa/1997/ast09dec97_3/

Radioactive elements increase the kinetic energy of their surroundings, as you pointed out, saying that radioactivity was one of the reasons that the inner Earth is hot.
I'm sure similar effect occurs in the atmosphere, after all, there has been many instances where radioactive atoms have been expelled into the atmosphere, think of all the nuclear bomb tests and the nuclear power station accidents.
http://en.wikipedia.org/wiki/Lists_of_nuclear_disasters_and_radioactive_incidents

From the above NASA site:

“Apparently, the particles that slammed into the ionosphere to paint the aurora borealis also energized enough atoms to head spaceward. (The same is true of the south pole where DE-1 made similar observations.) The acceleration mechanism is not fully understood, though.”

Perhaps radioactivity contributes to the effect.