"I’m afraid that I can only imagine the amount of air that would need to disappear in order to cause the observed effects, certainly it would be a huge amount."

Good. Now, why do you imagine no one has measured such a huge amount of atmosphere leaving Earth?

Note: If a significant amount of atmosphere were to leave Earth it could cause increased drag to satellites in LEO and thereby lower their orbits and bring satellites worth hundreds of million dollars crashing into Earth. Not to mention have serious affect upon ISS orbit. So you shouldn't imagine that such a issue wouldn't be important enough for some people to paid some attention to the issue.

"Earth is presently absorbing 0.85 ± 0.15 W/m2 more than it emits into space (Hansen et al. 2005)."

If Earth was currently "absorbing 0.85 ± 0.15 W/m2 more than it emits into space" earth would currently be getting warmer. And it's not, therefore Earth has not currently "absorbing 0.85 ± 0.15 W/m2 more than it emits into space".
Other than this is due to "natural variability" Hansen doesn't know why this is happened, but whatever the reason, Hansen would not claim that a planet which is cooler the present year from the last year, is "absorbing 0.85 ± 0.15 W/m2 more than it emits into space". Instead it's radiating some quantity more than it's absorbing.

"What if this extra energy is generated by nuclear power plants instead of being caused by the greenhouse gasses?"

What if, but how does this idea explain why in last year and over last decade, earth has not got warmer?

"How efficient are nuclear power plants? What happens to the nuclear energy that is wasted in the production of electricity? Why is the nuclear fuel more radioactive (energetic) when it comes out of the reactor than when it went in?"

Radioactive fuel is more radioactive because some of the material has a shorter half life. But the total amount of energy of the nuclear fuel fuel rod has been lowered.

Radioactive material in nature has reduced in energy each year that passes, over billions years this earth has less nuclear energy than at earlier times- this radioactive material was made from stars which supernova- heavy elements are not made from stars like our Sun. Everything in this solar system has reduced in terms of it total nuclear energy.
Something that decays because it's radioactive could decay into something with shorter half live and thereby emit energy in a short time period. Therefore the material give more energy in shorter time, and therefore can be said to be more energetic- but it doesn't gain in total energy.

So a lb of plutonium has a finite amount of energy. Any element I believe with higher atomic weight than Iron, can be theoretically in involved in a fission reaction and yield energy.

Thorium is example of this:

"Thorium (pronounced /ˈθɔriəm/) is a chemical element with the symbol Th and atomic number 90. As a naturally occurring, slightly radioactive metal, it has been considered as an alternative nuclear fuel to uranium.
...
Thorium is found in small amounts in most rocks and soils, where it is about three times more abundant than uranium, and is about as common as lead. Soil commonly contains an average of around 12 parts per million (ppm) of thorium."
Thorium, as well as uranium and plutonium, can be used as fuel in a nuclear reactor. Although not fissile itself, 232Th will absorb slow neutrons to produce uranium-233 (233U), which is fissile. Hence, like 238U, it is fertile."
http://en.wikipedia.org/wiki/Thorium

So, you add a neutron to Thorium and it becomes uranium-233. The half life of Thorium-232 is 1.405×10^10 years. The half life of uranium-233 159,200 years. So Thorium transformed into uranium-233 gets a shorter half life: 1 lb Thorium in 14 billion become 1/2 lb of Thorium and about 1/2 lb of some other element. 1 lb of uranium-233 in 159,200 years becomes 1/2 lb of uranium-233 and 1/2 lb of some other element.