Shanks literally talks to the Gorosei freely. If that ain't a red flag idk what is. Lemme tell you, he's definitely was not talking about Blackbeard to them, and I'm like 90% sure it's Luffy because otherwise Oda wouldn't need to have hid who it was after warning WB about him.
Shanks can talk to Gorosei
I know Oda
he want you to think that's a red flag
only to twist it and prove otherwise ...
Whitebeard was Shanks's enemy too .... talking to him is a proof that Shanks work with Whitebeard?