Weird main board issue, quite rare
Where did you buy your boards Zack? If it was ebay it's very possible you've received bad replacement boards. 3 things will cause no backlights, missing BL_On signal from the main board, inverter board issues (this is part of the power supply) or defective LED backlights. Your original main board has a bad EEPROM, the boot sequence is corrupt, this is why it's stuck in the loop and won't fully boot up. I'd test the backlight on command to be sure the inverter is being enabled, you should see around 3 volts at the backlight enable pin on the power board , if you see inverter enable signal the main board is most likely working ok. It's not common for the inverter section to give issues so I'd look at bad LED backlights as the most possible failure, if your able, when the set is working go to menu and back down the backlight brightess setting and see if the picture stays on, with it set lower the current draw will be less, if this happens it's bad backlight strips.