The first sentence is actually correct, but it is a little complex. The first part of the sentence ("How well they succeed holds enormous consequences for the world economy, ") means the level of their success will have a large impact on the world economy. "How well they succeed" is the subject, "holds" is the verb, "enormous consequences" is the object, and "for the world" is a prepositional phrase modifying the object. The second half of the sentence ("where China has emerged as one of the few reliable engines of growth") is a clause modifying "the World Economy". It is adjunct information describing the relation between China and the world economy to explain why the level of its success will have such a large impact. Hope this helps :)