I also have a PN4 nozzle with issues. It has not been touched for about 15 years sadly because of these issues and lack of knowledge at the time (I plan to restore it now that I am a collector)
The issue I am having is a lot like yours, only my problem is the brushroll refuses to stay attached and continuously falls off or stops spinning - either from the belt or from the shaft slots. I forgot what the issue was but I know that the brushroll will spin half a rotation or 1 or 2 rotations and then quit. I had a vacuum shop look at it in 2002(?) and they tried to fix it but couldn't guarantee it. I guess it fell off again shortly after because its just been in storage ever since.
Surface rust won't hurt the switch unless its totally rusted through. From spray on polish/cleaner and lint buildup on the reset switch its not uncommon to collect moisture. Those fuses are usually pretty bulletproof. Despite the issues mine has the fuse still works and its original to the nozzle. They are very sensitive and can pick up minute little resistances in the brushroll tension and cut it off pretty fast.
But I think your issue might be similar to mine just with different results.
