The idea that the ES charge is concentrated at one area on an LP or on the mat is contrary to data collected by Shure Corporation, when they studied ES charge in vinyl reproduction, albeit many years ago; they suggested that charge distributes evenly across an LP surface. However, I could perceive that there might be some assymmetry to charge distribution due to fingerprints or other oily or watery deposits on the LP surface. So perhaps the data you cite would contradict their earlier findings. I will take a look.
And we still have not had an answer as to whether the 18Hz high pass filter on the preamp was engaged during all of these investigations. If it was engaged, then I would expect there to be little to no woofer pumping when the platter is spun at speeds below 18Hz. Yet I thought the OP reported he can spin the platter by hand at speeds well below 33 rpm, and still visualize the phenomenon.