Concerning the first paragraph there is at least 3 items involved. The Ignition Switch, The Neutral Safety Switch and the Starter Solenoid. Of course all of the wire/cable connections have to be good also.

For trouble shooting I would hook up a remote Starter Button to the Starter Solenoid. If the Starter functions normal like that you may have an issue with the Ignition Switch or the Neutral Safety Switch (on an Automatic Transmission Car).
If the Starters Function is not normal when you use the Remote Starter Button (and all the wire/cable connections are good and clean; including the ground) you have narrowed it down to a possible issue with the Starter Solenoid or Starter Motor.

Look up the Neutral Safety Switch trouble shooting in the DIY Rpair section; if you think that is what it might be.

With the exception of eliminating the other causes of the starter not spinning there is no way that I can tink of to test the Ignition Switch.

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84 300D, 82 Volvo 244Gl Diesel