Complex sentence: Bernstein arranged the songs, and there was some uncertainty about Nyro's ability to lead the musicians by playing piano, so a pianist, Stan Free, was hired, and Nyro was encouraged to play the guitar instead which she rejected.
Split into simpler sentences: Bernstein arranged the songs, and there was some uncertainty about Nyro's ability to lead the musicians by playing piano. As a result, pianist Stan Free was hired, and Nyro was encouraged to play the guitar instead, which she rejected.