It's absolutely awesome that you've been helping people with this problem. I've had a similar struggle trying to get back a save file and unfortunately, I'm not one with any expertise in computers and I wanted to see if you had any advice.
Pretty much the problem was that I had a digital copy of Ultra Sun on an SD card. I downloaded ultra sun onto another SD card because I wanted a second save file, found out that I couldn't access my original save anymore and then deleted the new save that I made.
My game when trying to access the original save on my original SD card still gave me the message "The saved data is not compatible with this version, it cannot be used"
I found an article online that had a possible solution to this error message. He says that he used save editor (CheckPoint) to extract his main file - which was successful. Then he used PkHex to open it and modify his trainer information - specifically changing "Last Saved" to the current date and extending the length of the adventure by a minute. He overwrote the old save and it worked again.
I've tried to copy this as closely as possible but I can't get my save file to be loaded by PkHeX (It does work for my Moon save so checkpoint or PkHex isn't the problem). PkHeX says there's a problem with the file size.
Do you know if it could retrieved?