The Weeknd is about the only one of those I'd concede on, and even then he's been booked before on West Holts. Although that was a good while back and he's obviously a much bigger deal now. At this point there's been nothing from either side indicating that talks have happened and stalled. Doesn't mean they haven't - we just don't know.
Taylor, Gaga and Hazza will probably all do it at some point, with the Hazza being a very likely proposition next time he tours. I don't agree that Taylor wouldn't come back for purely financial reasons.
Bieber and Bruno the festival wouldn't be that bothered about going after. They'd be very weak choices.
They are definitely playing Berlin, not Munich and Cologne. Someone took the leaked dates for Manchester, London, Vienna and Amsterdam and added some ideas. Enough explanation?