Didn't Superstar say he and James were one being? So long as one lived, even if the other "died", they would eventually be reborn again? Maybe James was also given a letter by Bach, or James and Superstar simply counted as one Letter.
I think Mask is a manifestation of James' power. When James saw Mask getting beat AND losing his cool (cussing out Renji) James lost faith and died. Plus he was carved up into little pieces.