With enough effort you should be able to find a copy of the 940 green electrical manual online. Try this link
https://procarmanuals.com/pdf-online-volvo-940-1994-wiring-diagrams-tp3904202/
Starter system begins on p42. I've also seen the '95 manual available for download somewhere, although I have my own copy of that one.

Yes, that no/poor crank when hot and a good crank when cold is a bit of a puzzler.

You've checked and isolated just about everything except for the PNP switch, so as much as these aren't classic symptoms for a failed switch, probably best to eliminate that next. When the PNP switch goes you can often get it to work intermittently by leaning/wiggling the shifter forward/back/sideways in Park or Neutral if it's the contact area that'd grungy, but if it's the contact wiper block that's grungy (or worn) then those tricks may not help. While I'm thinking of it, check for back/forward slop in the shift lever in case one or both trans linkage bushings are gone. First sign of that is usually intermittent operation of the backup lights, but it can also affect starting, especially in Park.

Same is true if it was the ignition switch starting to go -lifting or raising the key while starting or forcing it over hard will often get it to occasionally work. Maybe reach up underneath and double check that the electrical connector is on good and solid and that none of the wires are notably loose in the connector.

If you hadn't said you'd replaced the starter then a sticky solenoid would have been my first guess. The ease of solenoid movement could easily be affected by heat and expansion. Once you get past verifying the PNP switch, I'd put that re-built starter back on my suspect list. The fact you can directly power it and get it to crank may be as you say, giving it enough of an extra kick to now catch.

Keep us posted as you've now got me extra curious about the temp side of the equation here.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now