I just had a quick look over what you wrote, I'm still coming down from work so my mind might skip or stray with ideas...
* With the first motherboard, Did you reset the bios via removing the battery or doing the jumper?
* Have you taken a laptop/pc outside and tested the display on a known working computer?
* Try the motherboard on a separate display and power supply?
When i have a computer that will not power up/restart/blank screen and so on, Will remove a single item and test it on another system to make sure that it does not do the same thing. Test each part if you have a spare setup as this may take awhile but generally is the best way to find a faulty part. 1 Hour and you can have tested a whole setup.
This means every separate item, Slotted Cards/USB Items/IDE/Sata/eSata/PS2/Serial/Parallel and so on.
Something like this to me sounds more like a bios issue or the screen..
Could be something like a faulty cable/plug or the screen is cooked.