So HP engineering said that this should work but they had not tested it. Make sure you are fully up to date on the firmware and the P410 should be able to go to the P400 safely. But.... caveat emptor, of course.

It was the motherboard. Tested the PSUs this morning with replacements and no issues with the originals. Swapped out the motherboard and the baby fired right up. We are currently recovering everything and anticipate zero data loss. No need to even fall back to the backups.