There is hope for bad singers!
When trying to sing or otherwise perform a song, there must be limit/border where the melody is getting so mishandled and/or the lyrics scrambled/lost it is not the song anymore.
It becomes not a performance of a song, but a reference to it, a hyper- or symbolic link. What remains is a whiff of the original.
When that happens, surely copyright does not apply, and performance should be free of any royalty claims from "rightsholders".
This is for (attempts at) performance, not covered by parody exceptions nor "X amount of seconds" rules.
There is a sweet spot to be found for the unharmonic singer as well as the forgetful lyricist! This should be valid for plebs and presidents alike!
Maybe it should even be an abendment to the U.S Constipation.
your humble non-excellent singer.