In the legendarium, mithril was also found in Númenor and Aman but it's become the unobtainium of this series. I don't recall it ever being closely associated with the Silmarils. In this show, Gil-Galad doesn't notice the rotting until after Galadriel had sailed away and the meteor passed overhead so I don't see how he can blame her. As far as he's aware, she's living happily in Valinor. He's going to get a surprise if she turns up in Lindon, but I suspect she's headed straight to the Southlands.