Just a random though, so I am probably wrong.
Have you tried harder changing BIOS to default settings? I have carefully read your post, but it is not clear to me if you have plugged in AC power to the system after changing the jumpers to clear CMOS. It is a required step. You removed the CMOS coin cell battery, I know, but have you drained all residual power from the system before putting the coin cell battery on the mainboard again?
I am suggesting trying harder these steps as some BIOS settings may be the source of a behaviour like the one you are seeing. It happened to me some time ago; in that case the machine had no obvious failures, no beep error codes and power button LED was white, so no errors found by POST, but it was booting only one of five times, showing a blank screen in other cases (just a blank screen, as there was a signal coming from both DP ports, slightly different to your case). That failure was caused by the Intel(TXT) enabled but not being provisioned in the right way —my fault, as enabling TXT locks NVRAM— before adding some hardware (a 16 GB RAM module and a 4 TB HDD, both original Dell parts for that workstation.)
You may have another chance to reset NVRAM by pressing Ctrl+ESC while booting until BIOS recovery menu appears. Right now I am not convinced you have a hardware issue, but it is hard to say without looking into your workstation.
Edit: the best choice would be going into BIOS setup, if you have a chance to turn on again your workstation, and return it to factory settings so you can discard some sort of assurance technology as the source of the behaviour your machine suffers. If you are able to turn the workstation on again, then I would run diagnostics on it too.