Kizaru dancing to the Nika beat was def a surprise for me too , well he has never been a tied down marine but i don't really think there's any specific signs for us to consider him in the straw hats ( tho it's def in the range after aokiji ) . As for his relation with vegapunk, they def seem more than just work aquintances but it's all in the realm of possibility.
also the negotiations being fronted by him logically actually makes sense considering how he's the g** of science but wouldn't the seraphims also carry buccaneer genes seeing how they're made from kuma ? was it worth the risk to bring something from the void century back in exchange for his physical capabilities? or did they just never consider joyboy popping up?
From what I know this is a non-issue for the Gorosei because the Pacifistas are devoid of personalities. They're just the prototype for the more advanced Seraphims and the Pacifistas are also on the weaker side considering New World captains especially Luffy and actual Warlords can one shot them so even if the entire Pacifista unit betrays the WG, they won't care.
As for the Seraphims they were made with Alber's genes instead that's why they have Lunarian traits. It seems that for both projects the Gorosei has the highest authority level so overriding them will be a pain unless Vegapunk has installed some kind of failsafe.
