Hi Glenn. I am glad you are hammering away at this problem and I am sure that eventually you will get this nailed.Back when I was a productive member of society I used to specify filters from Corcom and Spectrum Control in different design applications. My needs were TEMPEST related but these share many of the same needs required for general EMI suppression.
If it were me I would look for designs with dual stage filtering not just a single stage. They have filters that can start attenuating at 9 kHz. Also for differential mode noise what helps are filters that have capacitance directly across the primary lines. It is common to see an AC rated cap directly across the two lines on the input then a common mode choke followed by a split capacitance, again across the two primary leads. This whole assembly is then repeated by a second stage to increase the EMI attenuation. GL and thanks for the update. 73 Gedas W8BYA