does amanda change during the course of the play, or does the audience merely learn more about her?

No I don't think she changes. The safest conclusion to draw is that Amanda is not evil but is deeply flawed. In fact, her flaws are centrally responsible for the tragedy, comedy, and theatrical flair of her character. Like her children, Amanda withdraws from reality into fantasy. Unlike them, she is convinced that she is not doing so and, consequently, is constantly making efforts to engage with people and the world outside her family. Amanda’s monologues to her children, on the phone, and to Jim all reflect quite clearly her moral and psychological failings, but they are also some of the most colorful and unforgettable words in the play.