Ok, I'm back on terra firma now.
To answer the last post:
Lets assume that the aircraft depressurises, but does not descend. The cabin will depressurise to ambient. I might agree that if the cabin remains intact and there is some opening which allows some dynamic airflow into the structure, there might be some 'ram air' effect which would keep the cabin marginally above ambient pressure. I am not an expert on this subject, but my feeling is that the pressure rise would be undetectable on any display in the cockpit. You would not delay the descent in order to check that.
In the emergency descent case, following decompression at altitude due to a big hole, the cabin altitude would be expected to follow the ambient pressure during descent, less any lag due to the rate at which the increasing external air pressure could find its way into the aircraft stucture. All of your suggested minimal +/- differential pressure values would probably be unobservable during an actual emergency descent from altitude.
I remember from my distant 'A' level studies ( when it was a struggle to get even a 'C' grade) that there was thing called Boyles' Law. Something about P1V1T1=P2V2T2?? What that means , if I remember correctly, is that if a given volume of air (in a cabin) expands rapidly to a new and much larger volume, its pressure will drop and its temperature will fall rapidly.( Please correct me if I am wrong, I did chemistry in 1964). So, to answer the last question, as the cabin depressurises, the temperature will quickly fall below the cabin dewpoint and a 'mist' will appear throughout the cabin. As the air is sucked out of the fuselage, you would certainly be able to see the vapour as it travelled out of the cabin.
Referring again to Boyles' Law, the pressure reduction and the cabin air volume increase means that the temperature will fall rapidly in the cabin. Following a decompression, if the aircraft is not immediately descended below about 15000ft max, everyone on board without an oxygen supply would be unconcious within minutes and exposed to ambient temperatures well below - 35C . Yes, the cabin would get VERY cold!