(noformat) איש is "person", although it does have grammatical gender because all Hebrew nouns do. So it's true it can only really describe a man if it is applied to someone specific. Still, גבר is used when we want to be more specific or to emphasise him being a man and not a woman.
No. Usually, in the middle of a word it is pronounced "v". Sometimes, in the middle of a word, if it begins a syllable, it is pronounced "b", though. Maybe someone who is a native speaker or a more advanced student of Hebrew can fill you in on the ins and outs of when and why these changes in pronunciation with ב occur.