It's clear to me that the ballad originally had no chorus. There are a number of versions with a chorus. Linscott's version was covered by Paul Clayton and Clayton's version was published with his chorus as Polly Wand- attributed to Revolutionary War period. Dylan, I believe, covered Clayton's song.
Baring-Gould's English version from Sam Fone/J. Lukin published in 1905 had a chorus- probably of his own invention but based his two collected versions.
Several influential covers in the US had a chorus, for example, The Dillards in 1963, a bluegrass version. That same year it was arranged and covered by the popular folksingers Peter, Paul and Mary under the title "Polly Von." Frankie Armstrong covered Peter, Paul and Mary's version (see DT). Those two versions were the foundation for other new covers like a celtic-roots version which was arranged by Allison Krause with the Chieftains in 2002 and later as her solo version. Peggy Seeger did a cover version with a chorus using Polly Bond (collected from Rita Emerson, 1969) as found in Michael E. Bush's Folksongs of Central West Virginia on her recording Bring Me Home, released January 22, 2008.