What kind of junk energy is there to harvest from a car (in meaningful amounts)? I guess breaking is the obvious answer, but that’s already covered by regenerative breaking. Most car-based energy harvesting systems seem to employ speedbumps that clearly take useful (kinetic) energy away from the car (probably at a very poor efficiency).
No, they’re almost entirely unrelated. Almost all CPUs will idle close to 0 W (with correctly working drivers). The main idle power contribution comes from the mainboard and other devices (e.g. disks). The Mini PCs you mentioned should have a very low total idle power, probably below 10W.
Check out Wolfgang on YouTube, he has some great videos on the topic: https://youtu.be/Ppo6C_JhDHM