MSI B550M PRO-VDH completely dead (White CPU Light) - Ignores power switch and Flash BIOS aborts

The Original Problem: My PC suddenly black-screened while I was using it. When I tried to turn it back on, it got stuck on a solid white CPU debug light and wouldn't post.

Most importantly, the motherboard completely ignores shutdown commands. Holding the power button or shorting the power pins with a screwdriver does absolutely nothing. The fans just keep spinning forever until I physically flip the switch on the back of the PSU.

Troubleshooting Steps I've Already Taken:

• Ruled out the CPU: I took my original processor and tested it on a backup motherboard (MSI B450M Mortar) and it works perfectly. My CPU is safe.
• Ruled out other hardware: I tried putting a known-working backup CPU (Ryzen 3 3300) and a different working power supply into the broken B550M board. It still gave the exact same solid white CPU debug light and wouldn't shut down via the pins.
• Tried a Hard CMOS Reset: Unplugged the PSU and shorted the JBAT1 jumper pins for 15+ seconds. No change.

BIOS Flashback Fails: I decided to try forcing a blind BIOS Flashback just in case the sudden crash deeply corrupted the BIOS and locked up the Super I/O chip.

• I stripped the motherboard completely bare (no CPU, no RAM, no GPU). Just the 24-pin and 8-pin power cables connected.
• I took an 8GB USB drive, used Windows DiskPart to completely clean and wipe all hidden partitions, and formatted it to FAT32.
• I downloaded the latest BIOS file (E7C95AMS.2p2) and renamed it exactly to MSI.ROM.
• I plugged it into the dedicated Flash BIOS USB port (and eventually tried every other port out of desperation).
• The Result: When I press the Flash BIOS button, the tiny red motherboard LED blinks exactly three times and then completely turns off. The update refuses to run.

Is this board really dead, or is there a chance it is fixable? If it is physically fixable, is it even worth paying a repair shop to fix it, or should I just cut my losses and buy a new motherboard?