This is very welcome information!

However, a caveat:
Unless you enjoy litigation, bear in mind that the Carters altered many of the songs they sang not only to fit their particular style, but expressly to make their arrangements copyrightable--they were urged to do this by Ralph Peer, their agent/manager. Granted, A.P.'s copyrights often infringed on the copyrights of others, whether intentionally or not; that is now immaterial. In preparing a song, the Carters frequently changed melody lines, chord progressions and lyrics; these changes are identifiable enough that a court might easily detect the Carters' still-copyrighted contributions in your arrangements, unless you derived directly from original or p.d. materials.

In short, in most cases, you'll still need to credit the Carters as well as the original authors, and respect their copyrights, however questionable.