Well, there is a small hole in the side of the aluminum block that doesn't look like it serves any other purpose. Is the thermistor supposed to go inside that hole? It is touching the block right next to that hole, just not actually inside it.

I did some more monitoring of the voltage during a print and it did drop below 12. I'm not certain if I may have accidentally shorted something trying to do the measurements or not, but now as soon as I plug it in (before running any commands, no pc connected, etc) the hotend heats up until it reaches maxtemp then keeps going. I am trying to test it with an old atx powersupply with the green and black wire jumpered, but that measures at 12 v, so I don't see how that could cause this problem.

The vendor sent me new firmware, but it is a marlin.hex file, so I can't see what is in it and don't know what to put in the firmware file if I download vanilla marlin and flash that.

At this point I'm about to pull my hair out. I want to contact the vendor, but I don't know if I have a bad power supply, hot end, electronics board or all of the above.