Honestly, how can we be so sure that Lucci and Kaku are completely loyal to CP-0 and the WG, and don't have some kind of hidden agenda?
Remember, these guys were apart of the most skilled infiltration and assassination division of Cipher Pol.... After the events of Enies Lobby, the WG, dubbed them as traitors, and tried to eliminate them.
Nobody finds it the least bit suspicious that they would just so easily return to the World Government, after they tried to have them killed?
I wouldn't be surprised if Oda has different plans in store for Lucci and Kaku being here on this Island, than most fans believe. Oda is not in the business of recycling old villains, to be fought again.
So Imma call it now, and say Lucci and Kaku are only on Egghead Island under the guise of being there to eliminate Vegapunk, but they have something else planned up their sleeve.
Hell I might even go as far as to say that the only reason they came back to the World Government, is because they needed to get close to Vegapunk for some reason, and this was the best way to do so.
I could be completely wrong, but this is the best course of action Oda can take, because as it stands right now, it is kind of nonsensical that they are back with the WG, after they tried to kill them.