Hello I-O,
I understand you computer at this point will no longer boot, is that correct? I will try to help you.
With everything that has been done up to this point, I beliee it is ever the DC power jack or the motherboard itself.
The power connecter jack actually just plugs direclty into the motherboard, so it can be replaced without having to replace the entire board.
Also if you have access to a multimeter you can test out the power connecter jack to confirm that it is receiving the necessary voltage from the adapter. Here is a video that will show you how.
Please respond back at your earliest convenience.
Thanks