Okay, I think I solved my Sonar crashing issue ! . . . thank you to "microapp" who reported that there was a pop up message appearing when editing the Region FX of Melodyne Editor 2 clips using Melodyne Studio 4. This made me realize it is possible, so I had a closer look at my own system and discovered that my Novation Impulse control surface software interface was to blame for this crashing.
Novation uses the Automap software as a VST wrapper, so when the plugin scan happens at Sonar start up it renames the VST3 file. In this case it renamed "Melodyne.vst3" to "!Melodyne (Automap).vst3"
Apparently Melodyne Studio 4 is not compatible with Automap, or at least it doesn't like the renaming of its plugin file name . . . as soon as I removed "!Melodyne (Automap).vst3" from the VST plugin scan path . . . it all works as expected, and I get the pop up message when trying to edit Melodyne Editor 2 clips, saying "warning - - there's no turning back" (basically saying once saved, you can no longer open the Region FX with Melodyne 2)
Thanks to everyone for your inputs. Case dismissed.