I would doubt we're suffering the SAME issue (but who am I to know for sure? LOL).
Not only are our CPU / GPU systems completely different, the symptoms are also different.
You mention that on the 'vanilla' install you got the 'EE screens found...'
My 'guess' is that you copied across /etc/X11/xorg.conf (or one of the closely related X config files) from a prior install?
BTW, the method I used to 'disable' DRI is completely different to yours...
I created the following content in a new file (/etc/X11/xorg.conf.d/disableDRI.conf)
Identifier "Intel Graphics"
# Option "AccelMethod" "SNA"
# Option "Shadow" "False"
Option "DRI" "False"
With the 'vanilla' install, I also had to add in the 'Shadow' option to make it 'stable'.
With the drivers from /testing, I was able to comment out the Shadow option, and I elected to enable SNA (purely to see if it made a huge difference... Without DRI, the switch to SNA wasn't a 'quantum leap')