You seem to be saying that no one should profit from their version and recording of an old song, which is nonsense. If that was the case, no one would record it. The song might be preserved in dusty archives and the memory of a few itinerant wandering singers begging for change, but it would disappear from public view.

Do you have a referenced statement by Donegan saying he composed the song? Support your claims by quotes from Donegan and copyright holders, not claims by agents (by the way, this 'claim' is anecdotal- was it submitted in writing? Where is the record? Was the agent freelance? ).

Yes, the Lomax copyright (with the Library of Congress) holds on that version (two, I believe), just as Donegan (and his recording company) holds copyright on his version, Leadbelly (and his recording company) holds copyrights on his versions, and bakers-dozen others who have recorded the song hold copyrights on their takes on the song.   They all deserve their pay.