Well, at the risk of hanging myself again, I'll offer a few comments and questions.
I consider myself, primarily, as a lyricist. But I do write the melodies for my lyrics. I then send my very unfinished product off to a professional demo firm. In every case, they add professional musicians, or a professional production (when the "firm" is one guy who is good at computer music production). I pay for these services. I pay extra for the singer of my choice.
In the end, I get a finished product, and they have got my money for providing this product. If I am ever successful at getting a major artist to cut a song, they are entitled to zero.
However, if Polly, or anyone else, provides the melody (and note I didn't say the background chord progressions), she, because she provided a unique part of the song is entitled to a co-write. The arranger is only entitled to a co-write if an artist cutting the song uses his or her arrangement.
My opinion, corrections will be considered.
Ott