AFAIK the dark silicon term is specifically those transistors not always powered on. Doping the Si substrate to turn it into transistors is not going to change the heat profile, so I don't think dummy transistors are added on purpose for heat management. Happy to be proven wrong though.

My understanding is that pretty much every possible combination of these things is found somewhere in a modern chip. There are dummy transistors, dark transistors, slow transistors... everything. Somewhere.