Yes, Beyoncé has won two ASCAP songwriting awards. She mostly does vocal production nowadays, but she still writes some. But yea, her musicianship as a vocalist is beyond. Her vocal improvisations are even better. I just love her vocal arrangements.
Sorry, but Beyoncé is an actual musician. Not really up for discussion. She can write her own melodies and harmonies and has done so since she was a child. #facts.
Avril can write and compose, see most songs on Goodbye Lullaby and a couple of AL are self-written and composed (Remember When, Stop Standing There, Falling Fast etc) and she's also self-produced (4 Real). She can play Guitar, Piano and Drums to a pretty reasonable standard. I'd say yes, she is a musician.
Taylor is very much like Avril, she's a strong lyricist and can also compose her music. I'm not sure if Taylor has self-produced before? She's a VERY good Guitar player, and she can also play piano.
Kesha is a natural song writer, and she can play piano and guitar.