Portent alert. Does the foretelling that the power of the Eldar will
not recover the Sils spook Thingol? Why does she choose to remind him about
Finwë just now?
Well, one of the Silmarils gets loose via the deeds of a Sinda. The
chink in Morgoth's armor then lets the Valar do what needs to be done :)
--
Generally speaking, Thingol has tragic flaws, but I'm not sure that this is the
dominant one.
It's not that Thingol is unconcerned about the sons of Fëanor - it is that he
already dislikes them, and is glad to have them fighting his enemy even if he
is worried about them against him. I don't think that it is necessarily even a
tragic flaw. Consider: Melkor is in much the same position, having two sets of
enemies, Fëanor's sons being among them. However, Melkor I am sure is laughing
as the sons of Fëanor in their blind rage attack and destroy Melkor's other
(more dangerous) enemies! Melkor is defeated too, of course, but not by this
flaw. He is defeated because he underestimates the power of love, first
in Beren and Lùthien and then Eärendil and
Elwig!