"it does not sound much at all like a traditional shanty. Either it was only one line in the "old book" which was taken and expanded upon, or else the old book was written by a poet who never came close to a shanty."
Why not? - and to answer your other questions:-
1) He found it "complete."
2) Yes. No.
3) see 1)
What makes you think he was inspired by what he found in a book?
Who said it was a shanty (apart from Johnny Collins titling it)?
"no reason to believe it was traditional" - OK but what reason to think it wasn't? (shanty or no).