The way I read it, the "failure of nerve" refers to the fact Janeway didn't stay dead (e.g., make the creative decision to kill Janeway and keep her dead), while the "vindication of the narrow constraints" refers more to the faction that preferred series characters to remain inviolable - specifically, the people who were very angry about the fact that Janeway was killed in the first place and who were boycotting the novels until she was returned to life.
Basically, she's saying she knows that this story is going to be unpopular among the people who wanted Janeway to remain dead because they'll see it as reversing what was a bold narrative decision to kill off a Trek captain (at the time, apparently permanently, although with a back door in case she ever appeared on-screen.)
Meanwhile, she's also acknowledging that the people who were in the "boycott Voyager" faction are going to see it as a victory and perhaps try to claim that their boycott forced Pocket to bring Janeway back. She's saying that in reality, neither are true - it's simply that, whatever vision she has for the future story of Voyager
, she needed/wanted Janeway to return.
Perhaps she'll pop into this thread and clarify further.