Wow, you've been following up?
My bad. At any rate, if the percentage is even slower, that only reinforces what I was saying.
No. The surface and the atmosphere are different parts of the system, as the surface also can hold heat and therefore emits radiation as well. See Mercury. Also, IR radiations don't get "trapped" in the atmosphere. They are radiated from the surface, and a part of that of that is absorbed by greenhouse gases, water evaporation processes, and convection. Of that heat now present in the atmosphere, a part is radiated into space, and another part is reradiated back to the surface, which in turn absorbs it, starting the process all over again. I don't have figures for the exact proportions, but you seem to be better at finding those than I am.
I agree, however, with your previous speculation that CO2 would make the atmosphere unbreathable before any significant (and potentially catastrophic) increase of temperature took place.
No. That's negligible. Background radiation, solar wind, and even (non-cataclysmic) object impacts are insignificant as a means of delivering a significant amount of energy at a planetary scale. Consider Pluto, for instance, which is far enough from the Sun to receive substantially less direct radiation than the inner planets do. It's a frozen rock.