Yes, that information is useful and seems to confirm the sensor as defective.

A "1" on the display of the meter when the probes are not connected to anything means something like "over limit", so in that setting anything over 200 Ohms.

I always check the meter and probes first by firmly holding both probes together. The resulting reading should be very low, like 0.1 or 0.2 Ohms. Anything much higher and you must check the leads and connecion to the meter first to pevent false readings.

When all is well, then I would expect to see a much higher reading than 0.5 or 0.6 ohms through the sensor, even at 45 degrees ambient...uhm... ah 7.2 degrees C.
The way you describe the method of taking the measurement sounds OK to me, so I'm not doubting the values you measured.

Now I'm working from a 200-series here, but it seems like Volvo used the same sensor (Volvo part# 460191) from 1967 right up until the last 240s in 1993, so what I write below should apply to your car as well. However, I can't be 100% sure so anyone more knowledgeable, please chime in!

One of the tests Haynes says to do in the 240 is to shortly (max. 2 seconds!) ground the temp wire coming from the cluster at the sensor side and see if the temp meter in the instrument cluster deflects to full scale high (ignition on, engine off).
If it does, then wiring and instrument are deemed OK.
If it doesn't, then wiring and instrument must first be checked.

At such a low resistance as 0.5 - 0.6 Ohms, that basically equates to grounding the wire and I would expect to see a full scale deflection. This could be intermittent, hence the symptoms you described when you started this topic.

Since there is a warning to only shortly ground the wire from the temp gauge, to prevent damage to the instument, I would check the instrument too. If this erratic behaviour of the coolant sensor lasted long enough, it might have caused damage to it. Hopefully not, though.