Here are the only things I know:
as to the spelling, I think that the modifications have come from the difficulty -for the English and American speakers- to identificate its correct pronounciation (yakobatchee, like my e-mail address) with the way it should be written (Iacobaci, like my surname). I think my version is the oldest as -on the site nuovorinascimento.org I've found a poem by Pietro Aretino (an important poet of the 16th century from Arezzo, Tuscany, not far from Florence!) where he talks about a Iacobaci. It is a nice poem, written in satiric style (following the line of the realistic-jocose poetry -just like the great Cecco Angiolieri- as opposed to the precious stil novo and the troubadours of the precedent centuries), and he's not very kind to our ancestor, I may assure you!
I'm sorry for my bad English, I write better normally, but I'm very busy and I'm writing so fast...
Hope my informations have been useful to you