> By the way, Gandalf is contradicted in Unfinished
> Tales --I haven't got that text at hand and can't
> specify the section-- where it says that mithril is
> found in Númenor.
It's in "The Disaster of the Gladden Fields" -- more specifically in Tolkien's footnote 31: "For that metal [mithril] was found in Númenor." It's also mentioned in "The Line of Elros", in the description for Tar-Telemmaitë.
> There it says, citing UT, that the Elendilmir was made of mithril.
Yes, in "The Disaster of the Gladden Fields", the Elendilmir is described as "the white star of Elvish crystal upon a fillet of mithril." Following which, we have the footnote I gave above.