There is a reason that the laws are written the way they are. It would be impractical to copyright chord progressions. Melodies are unique. With chord progressions there are often many many songs that utilize the exact same chord progression.

As far as legal copywrite protection goes...I like what my old music biz professor said, 'court is not fun".

It is arrogant to think you need to legally copywrite.

No way will it ever come into play unless you are very well known.

And even then...it is still often NOT worth doing because of the legal costs.