Just to be sure that everything is correctly connected, you need to check this list, but before starting you should take a photo of the front panel connector then remove everything from the motherboard (you can keep the CPU). In the image below I highlighted the two connectors that need to be connected with the PSU and another one with the front panel connector: According to this manual which you can find more information about your PC, it didn’t mention the front panel pins labels, so assuming by default they use the same design as other MoBo you can compare this image with the one you took. The last thing to check if nothing works are the jumpers, I didn’t read the whole manual yet but could you please send us an HD photo of your Mobo.
Two things contribute to heat up the CPU, first a heavy load by a process and second a bad thermal conductivity, to eliminate the first possibility you need to make a backup file using iTunes then restore your iPhone to factory settings, try it for a couple of hours or minutes depending on how it behaves and if everything works fine then you might need to reinstall everything without using the backup, in case of persisting issue try to review your cooling CPU circuit.
I think the problem is coming from the HDMI port, sending wrong information, clean it with a compressor air, if it doesn’t work then you might need to remove it from the motherboard, but I don’t guarantee anything you can wait for other answers.
Ok, if nothing appears on your screen then you can say goodbye to your motherboard, but there are some limit solutions, the first one is to replace the BIOS chip with a new one, second is to find a professional that can re-flash your chip using special devices.