The NZ Folksong Page has DEFINITELY changed since I looked at it during the Wellerman craze. I'll skip what I remember but cant prove.

Not proof, but your suspicions about Colquhoun playing fast and loose seem to be borne out by this article referring to 'Song of a Young Country' as a "concept Album":
https://www.audioculture.co.nz/scenes/song-of-a-young-country

The "notes" surrounding the song are almost non-existent, but thee are crumbs in different places; Shortly after the LP came out, a double album version (and another 2LP in STEREO)with jacket copy and a booklet of liner notes came out. See nearly illegible pics of them on discogs under "more images."

https://www.discogs.com/Various-Song-Of-A-Young-Country/release/10275165/image/SW1hZ2U6Mzg3NDczNjE=

Finally, I looked in the "New Zealand Folksongs; Song of a Young Country" book and there is no scholarly information you are missing. In fact what is NOT there is made more tantalizing by what is. This edition is old enough that the 2 short paragraphs in John Roberts circulated scan are not in this book. There are NO scholarly notations and in the back of the book, the last two pages are tauntingly titled "references" and "Index and Sources." They are not.

"References" has a list of 27 short numbered entries mostly of an author and a title.
"Index and sources" is little better. The entry for "Soon May the Wellerman Come" tells us Page 10, words and music (coded "PD") "Public Domain with informant (not necessarily the composer) Frank Woods" and "Earliest published source this collection."

Also of interest to you it does NOT include any of the other reference codes which are:
"Arranged by, attributed to, composed by and copyright, collected by, refer to similar published song, (and most interestingly) Reconstructed by the editor from material collected."

Thanks again,
Walt