Hi Rollo, just a quick update (hopefully this helps others in the same situation).
I used this command to make sure everything was responding:
MCP2221a_iccmax_FF.exe -scan 20 7F
(basically asks each VRM “Hey, are you there, and what’s your ICC_MAX?”) — it confirmed both controllers (0x60 & 0x62) were already set correctly to FF / 255 A.
After that I reset the BIOS to Factory Settings from within Setup, saved and rebooted, and then went a step further with a full CMOS/NVRAM clear: powered off, unplugged AC, drained the board, pulled the coin-cell battery for about 30 seconds, and re-installed it.
I booted once with the known-good CPUs to let the BIOS rebuild its tables, then shut down and swapped in the 6259 CLs.
That combination of a factory-settings reset plus a full CMOS/NVRAM clear cleared the stale configuration data that was preventing the riser CPU from training. Both CPUs now come up and run perfectly.
So if anyone else runs into the same issue — main socket OK, riser dead even though the VRM mod checks out — try Factory Settings → CMOS/NVRAM clear before swapping CPUs. Worked like a charm for me.
CPUs were actually 8259Cl lol, boy these machines are hard work.