Nick had just killed Z when Diana made that statement, so she was not under Zerstorer's thrall anymore. The hole plot line in Back to the Future is that time travel can change the storyline – it is not a series of alternative realities. Both Diana and Nick knew what happened, just like Marty and Doc remember what the alternative future would be like if the past had not been changed.
A fantasy series about witches and werewolves is not meant to be based on science. Grimm was meant for an audience over the age of 15 and the ending was meant to provide a happy ending so that fans who had followed 123 episodes could get closure.
A fantasy series about witches and werewolves is not meant to be based on science. Grimm was meant for an audience over the age of 15 and the ending was meant to provide a happy ending so that fans who had followed 123 episodes could get closure.