Hi Andy,
Injected EDID into the dsdt and now I don't have to push the power button when it goes into sleep at boot, but otherwise behavior is the same. Upon booting, it initially goes into sleep for a few seconds, and now comes back to life with the logon screen. After booting, going into sleep manually produces the same scenario. After a few seconds it starts up again. However, shutdown does work. Also, the issues with the monitors remains the same. If the external is connected at boot, it's the only one seen. If connected after the logon screen, both are seen.
Any caveats with regard to the bios settings (other than of course AHCI) that might cause this?