Hayo
You asked for a response. Here it is but it's somewhat anticlimactic.
When I wrote my original question, all of my experience was with windows and just a few cursory tests on the Mac. It turned out that, due to my inexperience with Macs, I didn't recognize that it was behaving as I wanted even without the -R option selected. So everything is fine on the Mac.
With windows it was a different story. There were two aspects: Was it even possible to get the behaviour I wanted and if so, how do I set the appropriate options.
With respect to the first aspect, I know it is possible, I just don't know how. If you "right click" an image file in Windows File Manger and select "Open with RawTherapee", it behaves as I want, but I have been unable to duplicate this behaviour from the command line.
With respect to calling RawTherapee with various options, it was suggested that I could edit the registry. Well no! The other option would be to call RawTherapee with a Windows batch file that would call RawTherapee with the appropriate options selected. I tried that but then I discover that PM won't let me call an external editor that isn't an "exe" file. Since I didn't believe that I was going to get the appropriate behaviour even if I could call the batch file from PM, I decided to abandon my mission. In the longer term, I expect to switch my editing from Windows to the Mac and I can certainly live with the current behaviour in the mean time.
Bob