B How much atmospheric pressure is exerted on the human body?

You asked two different questions...

The average pressure exerted on the human body by the atmosphere is the average local atmospheric pressure. At sea level this is about 101kPa.

The average weight exerted on the human body by the atmosphere is the weight of the atmosphere above an individual. This is equal to the local pressure times the surface area facing up. I have seen the surface area on the top of a human head and shoulders estimated at .1 m².

This gives a weight of 10.1 kN, or just a little over 1.1 ton.

 

I suspect that you are looking for an answer to a question that has been given you or to settle an argument (?). The essence of what's going on is not really explainable in the terms of your question. It's really important to distinguish between Force (eg. weight) and Pressure (e.g. Newtons per square metre))
The 'weight force' of the Atmosphere on an object, acts inwards in all directions and is described as Atmospheric Pressure (which we all know). Our bodies are doing nothing particularly clever by balancing the inward pressure from the atmosphere with the internal outward pressure; everything does this. We have no strength to fight against atmospheric pressure so our internal pressure is basically the same as outside. It's the same for a mouse or an elephant; the pressures are balanced even though the total squashing forces are very different. Submarines, an exception, have rigid hulls which keep their internal pressure much lower than the outside water pressure.
The ambient pressure is 'right' for us (all animals) to breathe in enough Oxygen to keep alive and to stop us losing too much body fluid by evaporation etc. etc. But it's all to do with Pressure and not Weight of Atmosphere. In a space ship, the pressure is maintained with Pumps and doesn't involve the 'weight of a column of air'.

The net effect of the atmospheric pressure this is a very small upward Force. We experience Upthrust because we are immersed (Archimedes Principle) in a fluid (the air) in the same way that we experience (much more) upthrust when we are in water. The weight of displaced air will be something in the region of
-Volume displaced X density X g
= -80^Λ-3 X 1.2 X 9.8N
=about -0.9N, which is the Upthrust Force. (Equivalent to the weight of 90g of water)
which would be measurable - but not easily because we would need to measure our weight also in a vacuum and compare the two values.