Maybe there was more than one problem and fixing one revealed another.

If you have much use on the car and haven't changed the PNP switch - that may be a culprit - particularly since you say the car sometimes won't start but all the lights stay on (I assume you mean the headlights too) - this would point away from the ignition switch since the Volvo like many cars has a "load dump" circuit to disconnect loads just during cranking. I think the load dump may not work (as well as no current getting to the starter) if the PNP is bad.

If your PNP is marginal you may have other symptoms like erratic shifting, staying sometimes in 3rd gear ("limp home mode") or your backup lights not working. In my experience these problems were associated with cold weather and a cold engine. Eventually you will get a diagnostic code for it.

Did you try wiggling the gear selector WHILE starting? On a 240 with a bad gear position switch this clinched the diagnosis. On that car (not on the 850/s/v70) the switch was located where things could spill into it.