Ignored in all this chaos is Wenger doesn't rate Mustafi and wanted to get rid of him. He's staying because Jonny Evans rejected the Arsenal move.
So basically Wenger got desperate last year and bought a player he didn't rate. That's Arsenal for you.
Arsenal are going to have two unhappy players on their hands now too with Mustafi and Sanchez. That often doesn't go well. They got rid of Perez and Gibbs but they should seen through those transfers above and replaced them. Although I'm not sure why he doesn't rate Mustafi...