Without looking at a map showing the location of changes you can't draw any conclusion about the cause of the decreasing 850 mb trend between 1980 and 2000. There could be a period of drying in the descending subtropical highs, that is what the TPW data show. Whether you want to accept it or not, there is very good agreement between the re-analysis and satellite moisture data in the upper troposphere. There is also strongly increasing surface temperature, TPW, and ocean heat content and the timing is perfectly matched to man-made forcing with a big ramp after 1970. All well explained by climate science.
The science explanation makes much more sense than your "theory": a natural forcing which hasn't been identified but which is related in some way to the little ice age suddenly ramped temperatures in 1970 despite the absence of any water vapor feedback. Sorry that just doesn't hold together.