Maybe because the "bigger special fate" of the Halfelven would have overridden even Morgoth's curse? [Halfeleven seem to be in the "gray area" of the elvish and human fates interlapping and this seems to lend them a little more flexibility and leeway to manouver over some complex issues than any of the races has on its own each. A certain kind of synergy seems to occur in such unions - see their offsping e.g. Earendil and Elwing making it to Valinor while both Mortals and Eldar were banned to do so (after the revolt of the Noldor)]. So combined they seem to be able to "overcome" the might of the Valar to a certain extend and within certrain limits. This might have been leeway enough for Túrin, who as a human has a certain innate ability to "play with fate" anyway, to be able to "cheet" even the might of Morgoth to the extent to be able to get away at least from the utmost destruction.Question about Finduilas: how soon should we introduce her? Surely there's a reason why Gwindor says that only her survival can save Túrin from his fate.
Gwindor is also still in love with Finduilas so it is normal for him to overestimate her and her powers.
But actually she is in a compromised position. She has stopped loving Gwindor when he returns altered and aged after his captivity for the young dashing misterious guy. But Túrin is aware that in about 30-40 years he himself will look much worse than Gwindor does now due to normal human aging as such he does not trust her feelings to survive this if they did not survive Gwindor's diminished appearance. And Gwindor is his friend whom Túrin loves as such he is offended on his friend's behalf for her having stopped to love him. It makes all the sense in the world for Túrin to think little of Finduilas - from a mortal point of view. For Gwindor her - for an elf untypical - change of heart is a mark of a bigger fate overriding the "normal way of things" but for Turin is it just a proof of Finduilas' assumed weakness of character and lack of consistency - as it would have been in a human.
I think a cast Finduilas should be young but able to perform a inwardly"conflicted" character gracefully.