Ghost writing happens but usually artists with very large stature who are able to do it. Beyonce does it as well as a few others. Avril falls into that realm. Butch Walker definitely has probably been the number one reason for Avril's success.

Songwriters are loathe to give up their songwriting credits because that's where the money is. It takes a lot of leverage and cash to get songwriters to give up their royalties. Taylor definitely has the juice to make it happen but does she really need to do it?

My view is that Taylor writes her music and has evolved as a songwriter. Experience has definitely honed her songwriting abilities. I suspect a producer is responsible for the sound of the written tune being able to cross the genre line.