|1) Newly soldered and pristine power board.|
Take that, stupid machine.
The new power board is wired up and good to go!
Now all I've got to do is make sure nothing breaks, again, and figure out the source of the misfiring MP3's and I'm good to continue with the rules.
As for the misfires, I got two leads - interrupts (won't make the same mistake twice!) and possibly lack of pull-down resistors on the rx/tx pins. I noticed that sometimes the boards are restarting when many lights are triggered simultaneously, so it could possibly also be a power routing failure, i.e power taking a wrong turn somewhere, or not enough power from the power supply. The latter would however be unlikely as it should be able to provide plenty of amps for everything, so it's probably the actual toggling that somehow interferes with the audio playing.