Update:
It was failing to boot again this morning despite several attempts. After re-unplugging everything again and giving it one last try before preparing to take it for repairs, it booted.
After a lot of troubleshooting (extended hardware test checked out OK), I found that it won't boot if the USB cable running from the Apple Cinema Display is plugged in. It didn't matter which port, back or front (front tested with an extension cable), it wouldn't boot until the display's USB cable was disconnected. Anything else connected to the same USB ports wouldn't interfere in any way.
So... the problem seemingly resides with the monitor. Does this just seem really odd to anyone? Am I perhaps overlooking something? I'd hate to just outright replace the monitor, only for it to prove that it wasn't the monitor, but I can replicate the issue reliably as described. Any thoughts/insight appreciated.
