Flashing is ways cheaper than buying a replacement mobo,
Not necessary.
There are more than one member here got a whole used cMP. Extract the logic board. Fix the cMP. Sell the remaining parts. And end up get the logic board replacement for free / very little cost (or even earn some money).
It just depends on how you do it.
I've seen your post already. But you do not know what exactly died on your mobo. Could as well been a corrupted bootROM. By replacing your mobo you got a new bootROM as well that would have fixed it.
It's hard to believe that a working cMP will suddenly has a corrupted BootROM. Both Synchro3 and my case are similar. After a force shutdown, the cMP never come back. In my case, I command a reboot to Windows, but it didn't reboot after few minutes. I have no idea if already finish the shutdown, but no boot, to not shutdown yet. So, not 100% sure if that's same as Synchro3's case. But end up, I of course performed a force shutdown because the cMP didn't reboot to Windows as expected.
If fact, your case is very very similar to ours. You didn't perform anything firmware related (your GPU can't even perform any firmware flashing). It's just a normal OS update. But the cMP didn't reboot as expected, then you performed a force shutdown, then the cMP never boot again.
I really wish you can successfully recover your cMP by re-flashing the BootROM. If you make it. Then AFAIK, you are the very first one to prove that the cMP can suddenly has the BootROM corrupted with no apparent reason. And a force flash may able to recover this kind of sudden dead logic board.