1-The game is not buggy, this remastered version is completely remade. Your saves from the old non-remastered version won't work here. This remastered version is mainly for new players who haven't played the game before.
You can always press the 'CTRL'' button to skip through parts of the story.
2- She calls him ''Happy'' because she wants him to stop being gloomy and sad and find his happiness instead ( she's a kid operating with kid logic)
3- It's a running joke in the series to forget Nessrine's name or at least pretend to.