Alright Samwick. Here we go again.
First, I will concede that N2 does not seem to be a greenhouse gas. I too was surprised by that graph, but the e-28 makes sense. Good catch Samwik!
You are incorrect when you say that, "CO2's having the largest effect." CO2 does not have the largest effect. Water vapour has that honour by a long shot. This is mostly due to water vapour's vastly larger concentration.
As well, we did not visualize "the mechanism by which absorbed IR 'increases heat on earth?'" (Why did you put 'increases heat on earth' in quotes?) Molecules that absorb UV and visible light (and shorter wavelengths) are what can increase the heat of the earth. Looking at Canuck's link, the O2/O3 molecules will absorb UV rays allowing that energy to be converted to infrared heat through collisions with cooler molecules.
The absorption of IR radiation does not increase the heat on earth. It slightly slows rate at which the IR heat can escape. That increases the chances of it being reduced to a longer and less energetic wavelength, but it does not cause more heat. This has to happen for the energy to leave the earth since Canuck's link shows all energy from 0.2 to 70 microns, except in the ~8-13 micron band, being kept here.
Even so, increasing CO2 by 0.0002% from 0.0385% to 0.0387% will not have a large change in the amount that IR is slowed in its escaping our globe. Over 100 years, a change of 0.02% will have little effect since water will still have a many times higher concentration. You wrote that "heat is considered to be about 4-100 microns." Assuming that is correct, and looking at your earth observatory gif, water absorbs quite a bit between 1 and 3.5 microns. The only way for CO2 to increase heat on earth is through its ability to absorb energy at the 2-3 micron band. That is the only real chance for CO2 to warm the planet. Then again, it could also COOL the planet by converting the energy from the 2-3 micron range to the 8-14 micron range that can escape through collisions with other molecules. Unfortunately, water also absorbs in that 2-3 band, so CO2's effect will still be negligible.
