Guess that Shamrock was involved in Harald’s death (negotiations over Elbaf joining the World Gov, Harald either being naive or not the great guy that everyone thinks), and Loki’s went after Shanks by mistake for revenge. Shamrock teleports away, Jarul runs in and sees loads of dead people and Loki, assumes it was Loki’s fault. And that Shamrock can teleport back and forth from the castle because he’s been there before.
If Gaban is there -still not enthusiastic about it- it’s to explain Shanks background and God’s Valley.
I’m not too bothered about Shanks having a brother. It wouldn’t make much sense for Garling to just give up the Figarland heir to a pirate if there was no replacement. But yes, it is cheap how alike they look, down to the facial hair.