When you first disconnected the main board from the lcd panel did you do it with the battery out? When you plugged the lcd in to the new board was the battery out then as well. If this computer has an led lcd panel if you unplug the monitor cable from the main board or plug it in it can fry the motherboard.
Have you tried pulling the cmos battery out to reset the bios? Disconnect all other power or this won't reset the bios.
If you haven't already done this: take apart the lcd panel: take off the rubber screw plugs on the trim, unscrew, pull off the trim around it and unscrew it from the plastic back. Check that all cables that plug into the lcd panel are connected. Make sure you always disconnect all power sources before doing any work on any computer (battery, power plug etc.) you can leave the bios battery in.
Then recheck if you have any picture with bench test. Because you have a backlight on the lcd it tells me that it is getting power from the main board, it is possible that the cable was pulled lose when it was taken apart originally.
If it still does not work or does not post, I would start to suspect you have a faulty motherboard. Did you buy the board used? I know from experience that hp dv series and compaq g series had problems with built in video chipsets overheating and failing.