Now I have a couple more mysteries to deal with. First, I hope you deleted the output from the other 13 files in the first and last commands because otherwise I don't understand where it went.
Second, your 2nd command should work fine to change
the filemodifydate. I even broke out the Windows box
here and tested this exact command and it works fine
for me. Try using the -v3 option to see if that sheds
any more light on the subject (you should get an indication
that exiftool is shifting FileModifyDate, and a list of the
files that it reads).
Also, the -overwrite_original option is not necessary when
just writing filemodifydate since this is a "pseudo" tag which
doesn't require rewriting of the file.