The answer to your second question is something that would take a long time to answer, if there is a archive of the HZ going back several years, then you might find the answer there in the postings that I have done.

Your assertion that “There has been no overall long-term change in atmospheric pressure in recorded history.” May be in error.
Do you have peer reviewed evidence to corroborate your assertion?

For me to answer your third question thoroughly would take more time than the answer to the first question, but I will try to give a very short answer.

What I am talking about is the laws of physics, and the assumption that such laws hold true on a global scale as well as small scale in the laboratory.

Based on the fact that you have the letters PhD placed after your name, I will make the assumption that you have a thorough understanding of the GAS LAWS. http://en.wikipedia.org/wiki/Gas_laws
Also, I am assuming that the following statements are true.

1) “The average kinetic energy of a gas is directly proportional to its Kelvin temperature.”

2) From: http://science.nasa.gov/newhome/headlines/ast08dec98_1.htm
Solar wind blows some of Earth's atmosphere into space
“This shock to the magnetosphere excited the plasma trapped in Earth's ionosphere to a point where some ions gained enough energy to escape Earth's gravity and flow downwind of Earth. The amount of oxygen and other gases lost from the ionosphere amounted to a few hundred tons, roughly equivalent to the mass of oxygen inside the Louisiana Superdome”

3) From: http://science.nasa.gov/science-news/science-at-nasa/1998/essd20jan98_1/
“Thermometer measurements indicate that the temperature of Earth's surface is warming, while the satellite data above show a recent cooling trend in the stratosphere,”

4) From:
http://www.agu.org/cgi-bin/highlights/highlights.cgi?action=show&doi=10.1029/2010GL043671&jc=gl

“The remaining density anomalies (departures from climatologically expected values) began in 2005 and are attributable to lower-than-expected upper thermospheric temperatures and low concentrations of atomic oxygen near the base of the thermosphere.”

5) From: http://science.nasa.gov/science-news/science-at-nasa/2010/15jul_thermosphere/

"The density anomalies," they wrote, "may signify that an as-yet-unidentified climatological tipping point involving energy balance and chemistry feedbacks has been reached."

6) From: http://en.wikipedia.org/wiki/High-altitude_nuclear_explosion

Nuclear explosions amounting to more than 10 megaton equivalent of TNT has been exploded in the upper atmosphere.

7) From: http://www.atomicarchive.com/Effects/effects7.shtml
“A primary form of energy from a nuclear explosion is thermal radiation. Initially, most of this energy goes into heating the bomb materials and the air in the vicinity of the blast. Temperatures of a nuclear explosion reach those in the interior of the sun, about 100,000,000° Celsius.”

8) From: http://www.atomicarchive.com/Effects/effects14.shtml
“There are over 300 different fission products that may result from a fission reaction. Many of these are radioactive with widely differing half-lives. Some are very short, i.e., fractions of a second, while a few are long enough that the materials can be a hazard for months or years. Their principal mode of decay is by the emission of beta particles and gamma radiation.”

9) From: http://www.atomicarchive.com/Effects/index.shtml
“In addition to radioactive fallout there are long term effects on the atmosphere and climate.”

10) From: http://www.atomicarchive.com/Reports/Japan/Accidents.shtml
There are many nuclear power plants that contaminate the environment.

With more than 300 different fission products being released from each and every leaking nuclear power plant and each fission product imparting energy to the surrounding atoms and molecules, it follows that the surrounding atoms, such as air, becomes warmer.

The scenario then is as follows: Back in the years between 1958 and 1962 there was at least 17 nuclear explosions in the high atmosphere that heated the atmosphere in the vicinity to more than 100,000,000 degrees of Celsius. Such heating caused much of the affected atmospheric molecules to accelerate to faster than the 7 miles per second escape velocity required to escape from the Earth’s gravitational field. The molecules that were accelerated towards space are probably still going further into space. According to the gas laws, such loss of atmosphere was probably not measurable with instruments due to the effects associated with the gasses.
In other words, the loss of the gas pressure due to the loss of molecules was compensated by the increase in the temperature of the gas, therefore the measured pressure remained relatively constant.
Now after more than 60 years of nuclear pollution of the atmosphere and ground by various means, such as atomic explosions and nuclear power plant leaks, we are in a position to actually measure the effects, as seen by the information in #1 to 5 above.