I don't think a `foo feat. bar` folder is a problem, it's just how the artists were given credit by whoever published the recording.

> e.g. `/foo/foo feat. bar - title` rather than `/foo feat. bar/foo feat. bar - title.`

I haven't tried it and might very well be wrong, but I think the default script you suggested will produce neither of those because %_multiartist% will not be set for NATs.