Here is a link to a telephone interview with recorded music historian, Dr. Cristal Diaz-Ayala from the University of Michigan's Living Music site--It provides a lot of insight into the song and how it was performed in Cuba before and after Seeger--she addresses a lot of issues brought up here--

Inverview with Cristal Diaz Ayala

The song form a son montuno--Jand she says""The thing is that the verses that were sung with the
"Guantanamera" by Joseito were décimas. Those are ten-line poems with
a very complex rhyme, which is the espinela from Spain. On the other hand, the
Marti poems are just four lines. So actually Orbon had to do some changes in
the melody, in the harmon..."

She says that the song performed in it's original style was extremely popular, but only in and around Havana, and it was unknown in the rest of Cuba.   She credits Hector Angulo, Seeger and the Sandpipers, et al, for simplifying the song in a way that made it universal--and she doesn't call anyone a "thief"--