So we're back to Piazzolla! (it's close - but I don't know any other Brazilian composers besides Nazareth and Nepomuceno)
Monica... I have recorded some Miguez, Guarnieri and Oswald (the last one, you even said you enjoyed! hehe)
Regarding Latin America music, I have the impression that Brazil's music is too different from the other countries' (maybe because it's the only country which speaks Portuguese). Of course each country has its own culture, but I feel that there are a kind of "flavour" that seems to be common. Mambo, merengue and salsa seem to be similar dances, while Brazilian ones are quite different. they are mostly derived from the "maxixe" (samba, forró, baião, etc.).
as I'm not familiar to "Guia Prático", I cannot say much. I have to say also that the only folk song I have heard here in Brazil is "samba-lelê". all the others are unknown to me.
I listened to "Na corda da viola" and "Samba-lelê" only.
I think that "Na corda da viola" lacks fluidity, and the middle section's melody is not clear. "Samba-lelê" is a happy and dancing folk song, so I think it should be played more alive. but I don't know the score, so I can't say much. maybe Villa-Lobos wanted to change the mood of this song, like he did with Terezinha de Jesus (Terezinha de Jesus is a melancholic song in 3/4, but Villa wrote down as a dancing 2/4 maxixe).
Senhora Dona Viúva may be translated as "Ms. Widow" or "Ms. Widow Lady"