With regard to pitchers, don't forget Chris Bassitt. He pitched well in the Arizona Fall League (making the Western Division All-Star squad along with Marcus Semien) and while I don't think he breaks camp with the Sox, he will probably get a long look in spring training.
I also think that the Sox might keep Nieto as the backup catcher because of his upside and the Rule 5 requirement of staying on the major league roster. But I am not convinced that Flowers or Phegley will be the starter. Don't be surprised if the Sox make another deal or sign someone like Kurt Suzuki or John Buck.