If you analyze all the past e-mails and the issue description, I also think there is wiring problem, probably cut wires that act like a short circuit. Maybe this problem is at the origin of the old board destruction (motor voltage on the temperature sensor input). the short circuit could also be the reason of the brownout (voltage drop because of the current consumption). It is easy to check on the stepper motor wire if there is a square waveform on each pair. Anyway, we should send back the board to have a brand new one.
So, today's update :
I wired in a spare stepper directly and... Intermittent response. By that I mean sometimes the spare would lock up, others nothing at all, it would freely spin by hand.
So it doesn't seem to be wires. It's almost like it's not sending any signals? I'm honestly Confused. I didn't drag the scope over yet, but I think that's the next step. We need to see if the board is sending anything.
You want to borrow my handheld scope? There should also be a Fluke Scope Meter floating around somewhere unless it disappeared.