VirtualBox "Auto resize guest display" greyed out
I had a similar issue (didn't read all of your references though, short on time) and for me it worked with 3D acceleration enabled and grafics controller VBoxSVGA. Noting else changed.
I am on version 6.0.12 on the host and the additions are 6.0.14.
My solution was to set the display to VBoxVGA (VirtualBox 6.1.6 r137129 Qt5.9.5, Guest additions installed, Debian 10)