Replaced my CPU and got into trouble: My iMac won’t boot. When turning it on, three LEDs light up, but not #4. The LCD is connected though. I even had a different LCD connected, same story. Put the old CPU back in, same story. What could be wrong?

There seem to be different versions of the ribbon cable, visibly different at the screwholes - one is thinner with metal as screwholes, the other one completely black and the screwholes are more a part of the cable itself. Which one is newer / better / recommended?

What influence will the T2-processor have on repairing / replacing parts?

Can somebody tell me what kind of screws I would need to secure the new battery? I would like to put screws into the screwholes left open by those rubber grommets.