As far as I know, the Five Elders have always been aware of the real name of the Gum-Gum Fruit. The Nika Model of Human-Human Fruit has just been avoiding them because Zoan fruits have a will of their own. It’s possible that Shanks didn’t even talk about Luffy’s Devil Fruit at all when he spoke with the Five Elders.

Yeah i agree with @Atlas D. Aeradon besides they might have been the one to rename it to the gomu gomu to hide its true ability

@LucciXKaku4Evah they did rename it. They wanted to hide its true nature from the world.

Maybe

Feel like they could have figured from information shared by admirals and stuff

The pirate he wanted to talk about was Blackbeard not Luffy. It may be about Blackbeards lineage or something.

We do not know that