Try reinstalling the ram. Also is there a reason your rear fan isn't spinning upon starting with the others? Maybe that fan is dead? Unplug the fan and try that too.
So it's ok not to put screws in all the spots to hold the Mobo down? The hole under the 24 pin connector doesn't have one, is there a standoff behind it, idk just asking since I think you said you went through most of the stuff people put down