Q1: Why do think Thingol had troubled dreams of Men, even before he had heard
tidings of them. What kind of 'troubles' do you think he envisions? How do you
think he came to know of the Secondborn? Melian? Rumor from other Elven lands?
The Noldorin?
He has greater prescience and foreknowledge than many of the Elves, no doubt
because of the effect of Melian upon him. Thingol is able to sense the
troubled aspects of Men, but does not see the aspect of Men that will be the
salvation of the Elves (Melian alone seems to be able to sense that).
Q2: Do you think Melian refers only to Beren? Or also Hurin ?
This has to be Beren because she says he is of Beor's house. It is also
Beren who has the most direct impact upon their lives by marrying Luthien, the
Silmaril, and setting in motion the events that lead to the ruin of Doriath,
but also Earendil's journey.
Q3: Do you think the ‘doom’ is the parting with Luthien, the coming of the
Silmarils, the Fall of Doriath, or all of these? Other thoughts?
All of the above. They are the first couple to unite the two Children of
Iluvatar.

Nasmith, Flight of the Doomed (detail)