2) As you say, when more GHG is
added, the altitude of the
photosphere rises (I don't see that the tropopause would rise), and its temperature is reduced: so less power must be radiated away.
Instantaneously after
adding the extra GHG, the temperature profile of the atmosphere has not yet changed, so the upward radiation, which is sourced by the ground and the lower atmosphere, remains the same within the bounds of the old
photosphere.