There is one more thing to check, if you are willing to examine the file directly with a hex editor. The first four bytes of the d2s file have to be 55 AA 55 AA. If even one byte is different, then the file will not be recognized as a valid character file and it won’t show up in game. If it’s been changed/corrupted, then changing them back should at least allow you to see the character listed in the game. Hopefully the rest of the file hadn’t been corrupted.
Here is a picture of one of my save game files: https://i.imgur.com/SkE5q1A.png
Edit: further testing shows that the next 4 bytes also have to be 61 00 00 00.