Hi Peter,

When the starter refuses to spin with the key.
You can rule out a worn ignition switch segment and a set of alarm relay
points by trying to start with the service port in the engine bay.

Jump +12 volts from the battery to service port single female
spade connector (17/1 in the schematic)

There's a single green/red wire to the connector that breaks
out of the harness near the filler tube to the window washer tank
in back of the passenger side headlight.
Hook up the jumper to the positive terminal of the battery first.
Use a heavy wire terminated with a female spade lug and
do the jumping at the service connector which is far away from the battery
Don't make sparks near the battery, bad idea.

Then the only thing in the circuit will be the shift selector switch
to the starter solenoid. (assuming auto tranny)

With my old 740 the starter would quit dead with the key,
and It would start immediately by shorting the two solenoid lugs.
They were easy to get at with a long screwdriver.
Then the starter might work normally for months or quit immediately.
It was hard to troubleshoot when it was working.
I suspected the solenoid, but got rid of the car before trying.

Can you see the 850 starter solenoid lugs with the starter buried in there?
I never had an issue with my 850 starter so I don't know.

Strangely I've seen after market Volvo starters with the solenoid
for less money than I could find the solenoid alone, go figure.

Be sure to service your multiple Positive battery cable connections at the battery. Clean corroded terminal ends, check crimps especially on the smaller wires. Use Penetrox (zinc paste) or conductive grease on them: Nickel or Copper based anti-seize.

Good luck, Bill