I'll let someone else tackle the first question... I had a look at the details for the 747-400 IDG and got lost on the technical discription... something about spring-biased flyweights, hydraulic control pistons and wobbler plates. Basically, however, it sounds like it's using hydraulic forces to control the mechanical output speed.
Re your second question.... You will burn out the disconnect solenoid if you hold the switch in for too long. Basically the same for all IDGs. If it wasn't already completely stuffed, it now is (if you've kept your finger on the button).
Regards.
Q.