Recovering OptiPlex 7040 MFF, no video

Good evening everyone,

I've been following this forum for a while and have found great answers. I was hoping to get some help on my own project.

Short story: On a Dell OptiPlex desktop, I don't have video output. I got it for a couple of days after resetting the BIOS, but now I lost video again. Is this a motherboard issue? Is there a way to salvage the one I have, or do I need a new one?

Long story:
I figured with the whole quarantine thing and lockdown, I could spend some time learning by fixing an old desktop.

Where I would like to be:

Have a small, silent, power-efficient desktop permanently connected to my TV, mainly for videocalls with family.​

Where I started:

Dell OptiPlex 7040 MFF, i5-6500, 16 GB RAM, 500 GB HDD. No video on either of 3 ports, but did seem to boot up.​

Where I am now:

Back to no video, after having had video.​

How I got here:

(Some details and the order I did everything is a little fuzzy)
Removed all RAM, HDD, 3rd video port and fan, (after grounding myself with the metal part of the fridge and placing everything on a static-free surfacte) and dusted everything with an air can. I have not removed the CPU.
Reset BIOS, and got back to the "default" BIOS.
Got video back, YAY!
Next came problems 2 and 3:
#2: On boot, I got an "(A7) ME FW Downgrade - Request MeSpiLock Failure" message.
#3: Tried to boot into Windows, would not go past the welcome screen. It rebooted, asking if I wanted to perform a repair. Well, why not? I hit repair, but it did not work, even after several tries.

I upgraded the BIOS to the latest version, hoping to fix problem #2 (which I thought may in turn fix #3). After the upgrade, problem #2 remained. I was going to update the Intel chipset drivers, but they only come from Dell as a Windows executable file, so I thought I would tackle #3 first, then come back to #2.

At some point in time, I also ran the complete Dell HW diagnostics suite (hitting the F8 key), everything came back great.

For #3, it looked like booting Windows got stuck at classpnp.sys. Reading around, it was recommended to go into the BIOS, and try changing the HDD settings (to ACHI and then back to RAID). That didn't work.
I also downloaded a Seagate diagnostics tool, ran that for a few hours, came back with no errors.
I also ran "sfc /scannow" from a recovery USB, still didn't fix anything.

So I was set on doing a fresh install of Windows. Would love to discuss what version I can install with the product key I have, but that is for another day. Before doing so I thought it would be good to get the product key. I booted into a flash version of Ubuntu and got the key. I actually had Ubuntu on for a few hours, everything worked flawlessly.

Now that I was ready to reinstall Windows, I inserted the bootable USB drive, turned the PC on, and now I don't have video again! So I am back to square 1. What is going on? Is my motherboard fried? If it is, is there a way to salvage it? If not, would getting a new one solve the issue? I thought I would ask before resetting CMOS again and possibly making something worse.​

Thanks in advance for all your help!

 

Thanks WindowsGeek!
Before I get to changing the HD settings, I assume I will need to reset the BIOS again to attempt to get the video back.
Assuming I am successful again, should I update the BIOS? If so, should I update to the latest version? I am thinking I may go into the Windows registry wuth Ubuntu and figure out what BIOS was running before the crash, and update to that version.