I think you might be at your finger tips.

Position I is for the accessories. II is for running down the road. III is for starting or cranking. Position 0 is off.

Might be the switch itself or the connector on the backside? Power coming in is going open to the other circuits, not shorting. When the switch works properly the term “closing” sounds safer. Shorts blow fuses and burn wires for a “short” time!

The rear of the switch has a contact block that can be replaced from the backside. There are two screws that are rather hard to get too but possible. Connector or contacts can cause both symptoms.

You can cross the solenoid the battery lead to the solenoid terminal to crank the starter.

I think there is a wire that comes up by the firewall. It is on some Volvos. Check for it to come from the solenoid. You can use to crank the car.

Other posters will have more information or look in FAQ's.

Phil