Alda when I read that passage you seem to hold the same thoughts as I and the others here as to the origins of the dream. Though of note in the Index to the book is the following;
“Meneltarma Mountain in the midst of Númenor, on whose summit was the Hallow of Eru Ilúvatar (see Eru). 41 (unnamed in Tuor’s dream)…” (Index, Unfinished Tales)
It would seem that you are right to think it is the Isle of Númenor you see. Also it is said that;
“…when the Edain first set sail upon the Great Sea, following the Star to Númenor, the Elvish ships that bore them were each steered and captained by one of the Eldar deputed by Círdan;…” (Of Tuor and his coming to Gondolin, Unfinished Tales)
“the Star” if referred again to the Index under Eärendil is referring to his ship carrying the Silmaril. As for why he had the dream I do not want to be completely boring by agreeing with what has been said; that Ulmo sent him the vision. Why would Ulmo send him a vision of Númenor when he had previously shown him Valinor?
Osse would know..
