The biggest block to a merger would be the centrist functionaries who rule the NDP.
Nope, the biggest block to a merger is that consensus would have to be found at both the federal and provincial levels for both the NDP and Greens. The NDP is one party with federal and provincial branches. The Greens on the other hand have distinct federal and provincial parties and the former cannot make decisions for the latter or vice versa.
Can you see how complicated this would get? Makes the Alliance-PC merger look trivial.