As for Uranus being in Mariejoa , it's just a simple speculation :
Poseidon = ruler of Sea in greek myth , op's version is in in the Sea
Pluton = ruler of Underworld in greek myth , op's version ..... can be said to be in the Underworld ( at the root of Mt. Fuji , beneath Wano )
Uranus = personification of Sky in greek myth , op's version is in Mariejoa ? Since Mariejoa is the most "Sky-like" place in op
Wouldn't be surprised if Imu is Uranus , Gorosei bowing down to "personification of the Sky" while not giving that much shit to Akainu/Shanks makes sense .
Could be wrong of course . From my experience , using Pattern Arguments like this sometimes failed , sometimes succeded .