Mark, I don’t have any expertise at all in this area but, using some reverse logic, it seems the pi is getting the configuration on boot from somewhere, obviously not the cmdline.txt file. Could it have been cached somewhere else in another file, driver or module perhaps?
I noticed that the font size of the boot messages is smaller than the normal std Raspbian boot messages, so could it be that the output config settings like font, framebuffer etc… are being picked up somewhere and used early in the boot process? I realised this suggestion is probably ‘rubbish’ but then something odd seems to be happening here.
Is there some work-around that I could try in an attempt to isolate the issue like, re-directing FB output to HDMI prior to startx??
My Karma just ran over your Dogma.