Bart R
Let me answer your question from bottom back to top. Yes, we can draw many layers of onion in the atmosphere, with equal optical thickness (99% absorption). But how much IR leaves the onion is determined only by the outmost layer, which is the “surface” and the rest are “inside”.
All the CO2 molecules emit 15 um IR as long as the temperature is not 0 K. In other words, every CO2 molecule is a radiation source for the 15 um IR. Therefore, wherever within the onion, there is the 15 um IR radiation, “scattering and backradiating over and over again.”
Your onion model may be useful to help explanation. The inner most layer absorbs the IR from the bottom by the earth ground surface, and the IR from the top by the inner 2nd most layer. The inner 2nd most layer absorbs the IR from the inner most and the inner 3rd, and so on and so forth until the outmost layer.
There is no countable CO2 absorption in the thermosphere. Therefore for the 15 um IR, the outmost layer of the onion starts somewhere below the thermosphere downwards.