I like to think about Monet and about PH this way: Monet doesnt think CC as a great cientist. Furthermore, she mocks him as you can see here. And she doesnt seem to expect much of CC's work. Ok, she calls him master, but that should be just a facade. CC must be just a puppet, he is this new guy's puppet, and Monet is this new guy's right-hand man/woman. CC doesnt know what Monet has been doing neither that this new guy has come to PH. All of that happens "a sus espaldas" (as we told in spanish; behind CC's back, or with CC ignoring it). On the other hand, she has been guarding CC and Law as well. And she has been doing a great job, since a so astute strategist as Law didnt notice her role at all.
But mainly, she has been guarding PH. I mean, if PH events are important, they are important not because of CC's work, but because of that important key Law is searching for. And all that poison in that wild and legendary island, all that slime is just a facade to preserve important key until the right moment comes.
But why is this new guy in PH? Or he is the important key himself, or he wants that important key for him. In the begining, CC and his bizarre circus, that includes kipnaped kids from around there, were a more or less convenient facade. A facade that required a minor intervention: creating fake news for Marine and WG… Then Law came to PH and CC+CC's circus+Law turned into a not so stable situation, but something that should be borne/withstood… But with Luffy and SHs adding to the equation, that situation requires an urgent intervention: or in order to preserve important key safe in PH or in order to get it once and forever.