Do you have a device that at one time had the working CSV that you can lay your hands on? If so, install Files, navigate to the ODK folder, find the project / form and go back through the versions folders and take a copy of the CSV from here.
\org.odk.collect.android\files\projects\[yourprojectID]\forms\[yourformname]_[version#]-media\
Or connect over USB and browse from PC...
It's likely not your solution, but I did have an issue where excel was creating a slightly noncompliant CSV and I had to add a 'false' first column - header was 'a', no data in the cells below it, then my second column was 'name' etc etc.
And I also sometimes had errant cells that seemed empty but would cause problems, so going to the last column then selecting the next column to the right, then extending all the way to the end (ctrl-shift-right) and deleting, then going to the last row, selecting the next row below and extending to the bottom (ctrl-shift-down) and deleting would clear these out also.