I liked the ending, because it wasn't the cliché of a happy ending.
In my opinion it means that the way to self knowledge never comes to an end. April has to go on...
About April's father:
Spoiler:Doesn't matter how often I play the game I am still unsure if it was "only" physical violence or also sexual abuse. What do you think?
The importance of 13?
Spoiler:Lady Alvane tells us that the 13th guardian is the last one. We had 13 chapters to play...