We've reached as far as we can go without swapping out components - it could be anything - but my money is on the motherboard or CPU as light indicates a problem.
The problem we have is that you took apart your PC, without taking any anti static precautions which can damage/kill components (haven't mentioned before as may not be relevant). Also, you had tried to boot the system many times with the cooler not making proper contact with the CPU which may or may not have damaged it.
It could be the PSU or GFX - nothing is impossible - but given the above, i think the MB or CPU are the most likely affected.
EDIT: You seem to agree