As for copy protecting newly arranged or modified PD material, it was common practice for record companies to do that when their artists recorded PD material. It is not always done today.

I have done so for songs that I made significant changes to, for example used old melodies for entirely of mostly new lyrics - or for putting a PD poem to music. But even when I've rewritten whole verses in the style of the old song I've listed the song as PD.