Answer to Question 1
It is not only desirable, it is essential. If we cannot show that something is
incorrect, we can never tell whether or not it works at all. But science works by discarding
inadequate models. It is to be hoped that the student answer will include this point.
Answer to Question 2
Imagine the molecule of carbon dioxide sitting in the atmosphere. It is bathed in
longwave (infrared) radiation from Earth below. It can absorb that energy and become
excited, then radiate that energy away. When it reradiates the energy, it can and does emit
the radiation in all directions. So half of the radiation ends up going roughly upward, half
roughly downward. This occurs for each molecule, so the net effect of all the interactions is
to cause part of Earth's outgoing radiation to be trapped.