The file system shows as RW if I mount a symlink to the location. But as soon as I start to write to the location, it switches to RO
Never mind. I got it working last night.
I have to say... Reverting back to the internal storage has made things much snappier and smoother. Which is baffling, because the results of my Read/Write tests of the internal storage vs the external SD card indicated otherwise.
Thank you KOPRajs for providing this backup approach for if/when the internal storage begins to fail. I will keep this in my back pocket.