Jasz:
all my sketches were in "arduino-1.6.13"
Don't ever put sketches or libraries in the Arduino IDE installation folder. Just leave that folder alone unless you have a very good reason.
Jasz:
Tonight I tried again and all 4 instances of the Arduino IDE has retained
File -> Sketchbook location as
/home/user/Dropbox/arduino_sketchbook_BACKUP_COPY_24.10.2017But It will not show the directory when I navigate to
File --> SketchbookInstead it shows my old sketches
Sounds like a dropbox issue, not an Arduino IDE issue.
Jasz:
I have no idea if it backed these up somewhere; I deleted the old sketches directory.
It's easy enough to find out what's going on: Open one of the sketches in question, then do Sketch > Show sketch folder.
Jasz:
This rather clunky aspect of the Arduino IDE, I find, hampers creativity significantly.
What is this "clunky aspect" you refer to? You pick a sketchbook location and set that in preferences and then that's the sketchbook the IDE uses. What's clunky about that? It seems the only logical way to go about things to me. Of course you can store sketches anywhere you like. They just won't be accessible via the File > Sketchbook menu if they're not in the sketchbook but that's no big deal, you just have to open them via File > Open or by just opening the sketches directly.