So either Shanks is a holy knight or he has a twin
I really dont think the twin shit works. The Gorosei still questioned why he was there to visit him. If he were a typical WG employee, then they wouldnt question it.
Really only leaves 2 options:
1. A separate, different looking Figarland member and Shanks still has access to the Gorosei for his own means
2. Shanks is the God Knight but does not serve at Mariejois