Gary, we agree on everything, but your statement, as written in #5 is simply not true UNLESS you also include your assumptions! Really not trying to nitpick, I swear! You can't just make a statement like that..It is probably an oversight, but is really misleading as written. You must state what material you are heating to 126 Degrees C to equal 1.2 bar and how much free air space existed. If you want to say that the E39, when filled to the correct COLD level and containing a perfect 50-50 mix of anti-freeze will be at 1.2 bars of pressure if it reaches 126 degrees C then you have a possibly valid statement. I trust your math, but you can't make a statement without specifying the liquid involved and the air gap. Like you pointed-out earlier in a way-air can compress, a liquid can't. Water can only compress if it changes state; like from a liquid to a solid.
The single most compelling reason in my mind for the lower pressure cap is for those that over-fill their reservoir on a regular basis. Then you stress the entire system to 2 bar to let the excess coolant out so the system can return to under 2 bar. If you fill the system full, the pressure in the system will exceed 2 bar and release ONLY enough coolant to get the pressure back to under 2 bar...say 1.9 bar for arguments sake. The next time you start and run your car, the coolant level will still be too high and the pressure will likely not reach 2 bar a second time, but may reach 1.9 bar quickly and stay there, in theory, forever, or until you somehow lose enough coolant to create the proper air space. For this reason, the lower pressure cap makes a lot of sense, but not to control your temps or what your cooling system wants to do. It is only an advantage for those who A) like the look and B) over-fill their systems regularly.
A system will only be at 1.2 bar @ 126 degrees with a very certain liquid in the closed system. I seriously doubt that it is possible to know exactly that you are at 50-50. Maybe close, but you never have a completely bone dry system to start with. Let's agree to not mix pressures and temperatures- just to know the relationship and the properties of the liquid in the system to make "assumptions". I really appreciate your testing and your thoroughness. Your statement was obviously making some assumptions that some might not get while others would. I especially like your objective evidence of the pressures observed with the proper fill level and the right mix of coolant. The car never exceeded even 1 bar. That was enlightening to me and thank you for that. Beautiful piece BTW! definitely FTW. Link to order please guys.