In case this helps anyone avoid the amount of wasted time we've lost on this.
Everything works absolutely fine in FileMaker Pro!
In FileMaker Go (19), when on a layout using a button to start the calling script, passing the callback script name as a parameter, everything works fine - providing the web viewer is present on the viewable layout and not hidden using 'hide object when'. This is not necessary for FileMaker Pro, which will work with the web viewer located in the grey offscreen area.
In FileMaker Go Running the same calling script, passing the same script parameter from another script fails to launch the callback script
I would estimate around 6 to 8 hours of lost development time due to the above and would like to publicly thank my wife Val for her suggestion to finally resolve the problem.