You can't get the Iceberg Key beacuse you can't catch Regice in Black 2.You can catch it only in White 2.
In White 2 you can catch
Regirock and Regice, NOT REGISTEEL
In Black 2 you can catch
Regirock and Registeel, NOT REGICE
So we conclude that Regirock can be caught in both games.Regice can only be caught in White 2 and Registeel can only be caught in Black 2
Source: I have White 2 and I've caught Regirock and Regice