> As an editor wouldn't you try to find other [oral] versions in order to add stanzas?

Probably not. Songsters were produced for cash, not scholarship or even for the serious collection of orally transmitted songs.

Apart from that of a few academics, there was little 19th century interest in oral tradition as an object of general study.

Baring-Gould was the leading pioneer in taking folk songs seriously, but even he had little respect for the verbal integrity of what he collected or published.

Were an editor already familiar with an additional stanza, he might well have included it. But going out of his way to find "traditional" lyrics? Extremely unlikely.