SUCCESS!
After following the excellent guide by dsanke here, I finally got it working!
All I had to do was:
- Swap the Initialized ME region with an Uninitialized one,
- Edit a few lines in the XML file as explained,
- Flash the newly built outimage.bin.
Now my OptiPlex boots perfectly and full restart/sleep functionality is working!
Big thanks to dsanke and the whole community for the help! 