Hi Bill,
I'm very familiar with that bug, I helped test the fix that was installed for 10.6.1 - I also helped write the TLS Whitepaper you posted (thanks for promoting it!)
The add data>from portal issue when the server only supports TLS 1.2 should be fixed in 10.6.1 - we're working on patches for previous versions.
Can you confirm your Desktop versions and the TLS versions you're supporting on your portal? ArcGIS Desktop 10.6.1 connecting to a portal that only supports TLS 1.2 should work (my personal portal only supports TLS 1.2, and I run Windows 10 as a client). Seems there's more to this than that bug.