I don't know why we have this obsession with differentiating between 'Alton Towers' and 'Merlin', which crops up on here over and over like they are separate entities. They are the same thing. People working at Alton are Merlin employees, they represent and work towards Merlins quite tightly defined goals using Merlins very tightly defined budgetary constraints using centrally published policies and processes. Anything done is therefore a product of Merlin, regardless of whether person making it works at Towers or in Poole.
Even if the Melors contract was put in place locally without anyone working externally from Towers noticing (which considering the size, cost, and reputational risk involved is unlikely in the extreme), the procurement process would have followed well defined Merlin dictated policy and procedure and decision flow chart to justify. It is therefore a Merlin decision regardless.