I’m gonna say Jet Force Gemini. It’s not hard in that enemies or bosses are difficult, though some were. It was those damn Tribals. You had to save every single one of them if you wanted to beat the game, and some were a pain to save without them getting killed.
From what I remember reading, yes there is some added content from the books that is not in the musical stage adaptation. I think it’s more of Elphaba’s childhood (which is quite a bit in the book).
Also remember reading that one of the reasons they decided to split it was because they couldn’t find a reasonable way to move on after “Defying Gravity” (This is the big number before intermission in the musical and really does feel like the end of that chapter) to the next scene in the film, so they decided to split the films too. Not saying it was the best way to go but maybe it means they can add more book related stuff to the second half too. The musical is sorely lacking there too.