The chanter is quite similar to Galician Open chanter, in fact some makers uses Galician chanters....BUT... Diego Piñeiro makes a reproduction of and OLD Xeremia Chanter...and this is different from the moder ones, maybe without crhomatic notes but with the ancient flavour.
The modern chanters can oberblow the same as a Galician one but...this is not important. You will not overblow anything because the repertorie from Mallorca don´t have songs with second octave notes, and if you want to play the second octave, you will find some problems using three drones at the same time.
Tratidional keys are Do and Brilliant Do, but Diego can makes new Sib Xeremias, and he has made a kids version in Re. Very funny and very cheap (450 euros).
About Baines theory....I don´t know...Maybe the Xeremia is the son of the Sac de gemecs OR maybe the Sac de gemecs is the son of the Xeremia. Ancient Xeremias are more elaborated instruments with better appearance than Ancient Sac de Gemecs (though the tenor drones are dummys in some instruments), so it´s possible the Sac de Gemecs it´s a devaluated version of the marvellous Xeremia.
Anyway, I think that the appearance of the Xeremia and the Sac de Gemecs with the drones in this way has a good sound reason: of course, it has sound benefits, but I think the main reason it to have something similar to Italian Zampognas in appearance.