1. The Karpeles definition (1954) allows for the adoption of composed tunes as much as it does songs, in my view, by due process of folk!
2. It may well be that musical literacy (ie the ability to read music) was much more common 100 years ago than now, and who knows about 200?
3. One can only clear a tune (or song) via the MCPS if the copyright owner (or his/her music publisher) is an MCPS member AND it is not subject to the first recordign procedure: a member may list a song or tune at MCPS and specifiy that it may not be licensed for first recording without his consent. In some jurisdictions eg Australia AMCOS has a more sophisticated procedure by which a copyright owner may exclude specified things for specified songs from the liensing agency's power.
4   The MCPS mandate does not generally include significant alteration or the change of words. I was once involved with a situation in which a TV company thought to produce and educational video of a well known modern classical work - did the bits with the MCPS and then recorded it with the movements in the wrong order. The composer's estate hit us like a ton of bricks!