I gathered from the book that to some extent Ashley's promotion was slow because she was a slow(er) developer. I think she also mentioned that as she was rising her rival Colleen Neary was struggling, and Balanchine may have delayed her promotion so as not to demoralize Neary until he was certain Neary wasn't going to bounce back.
He did tell Ashley initially that it was Mazzo's role, but soon after Mazzo was out again and Ashley was cast, and thereafter they shared the ballet.
(Not disagreeing with your general points, I should note.)
And of course when he was ready to show you the door he didn't hesitate to take roles away. For Violette Verdy the last straw was when he gave "Emeralds" to Christine Redpath, explaining that Redpath didn't have enough to dance.....