Instead of variables for the path, you can use string constants and an absolute path. So instead of
%_folderpath%\%_filename%.txt
use
\\?\unix\Volumes\Music\PNTemp\%album%\%_filename%.txt
And no: covers cannot be exported with the text export. You have to use an action of the type "Export cover" and enter the required combination of variables and string constants to save the covers in the correct folder.
How can I import that? I have tried using
\\?\unix\Volumes\Music\PNTemp\%albumartist%\%album%\%_filename%.txt
and
\\?\unix\Volumes\Music\PNTemp\%albumartist%\%album%\tags.txt
but it won't work
As there are no variables set for reference, you get an invalid (i.e. not matching) filename.
You have to use a (fully qualified) filename without variables.
Depends.
If you create one export file for all files in the collection and you have the filename as one part of the record then you can import the data from 1 export file for many (selected) files.
If you create one export per folder for all the files in that folder, you can at least import the data for all those files.