The final decision for Mutiny Bay was not made until Summer 2007. As a result, it surely must fully be with Merlin that approval for the attraction(s) were given.
They may have approved it but not planned it?[/quote]
Well, Tussauds/DIC wouldn't have "planned" it either. The planning is very much down to what the management at Alton Towers want to install - Merlin may give them an overview of the style of rides they want to see at the park, but it is the management that discuss and decide on the final things.
Merlin/Tussads etc. merely stump up and approve the financing.