Have you tried to see if it will load the pre boot assessment? I think you hold the Fn key while starting the machine until it pops up but I can't remember for sure.
If you haven't already, you can try some simple things like unplugging the power adapter and the battery and holding down the power button for 30 secs to drain all power. Then just plug in the adapter and see if it boots. 7 beeps usually means the CPU but if you've replaced the board and CPU then something strange is going on.
If you can get hold of MrFox on here or anywhere (maybe try his MrFoxRox youtube channel), he is the M18x king of the world and if he doesn't know then you're in trouble.