Well, Q, I looked at the U.S. site you recommended. It is a good one.

However, until the day someone develops software that lets us search for a melody, it will be impossible to determine accurately whether a tune is copywrited or not.

As for determining whether the arrangement is copywrited or not, I don't think it will ever be something that a person can pin down. Fortunately, folkies are adept at generating so many permutations on a set of chords, that I don't think it's an important question.