Ordinarily calibre handles all filenames itself using author/title/calibre-id. This shouldn't be a problem as you should send using calibre. Unless e.g. you sync to Dropbox and want to downlod via the DB web interface -- so I quite understand where you are coming from.
As theducks said, set the tweak to ensure default author == LN, FN.
To prevent metadata download from overriding
your custom author values, select the option to Configure metadata downloads, and uncheck author from the list of metadata columns to update.
theducks -- a bit harsh I think.
