That's a good guess. If it was only the windows, I'd be thinking that someone did some wiring to make the windows work with the key out. But if the fan continues to blow, well that's an indication that somethings wrong. The ignition switch is a moving part and wears out....so that's a great place to start. You know, that could be part of your tach problem too. That's a really long shot though. You might not be getting enough current. Did you measure the voltage at the tach terminal? When cranking you should get more than seven volts. At this point, I'm throwing darts in the dark.
I'm like the blind person throwing rocks at a coffee can a hundred yards away. Given enough rocks, I just might get one to go in.