Curses!

[Nilpaurion Felagund]

It makes you wonder, did Túrin's curse had any real effect on the Ruin of Doriath? Or was it just his bitterness towards them for the fate of his sister and mother.(Maédhros)

Perhaps both. After all, Túrin was devastated at being the cause of his sister's death, and has this terrible habit of lashing out against those who bring him closer to his Doom - Brandir was a good example.

And the Nauglamir, one of the causes of the ruin of Doriath, was delivered by Húrin from Nargothrond, as "payment" for the "fair keeping" of his son. Túrin killed himself due to Mablung's words. Húrin saw that, and probably was angry at Mablung - and Doriath, for many other reasons aside from this. So he adds this little item to his "Things to do once I escape this place" list:

~ Bring something to reward the king of Doriath for "taking care" of my family. (If it wasn't for him - and his subjecs - all of them would still be alive.)

But another thing I ask here...who effected the curse on Húrin's hîn? children, I mean. All evidences point to Morgoth - through Glaurung - but...

[Húrin: ]...you cannot see them, nor govern them from afar, while you [Morgoth] keep this shape, and desire to be a King visible upon Earth.

(UT I 2)

[Tuor of Gondolin]

And what of Eol's curse on his son (twice):
" 'Come, Maeglin son of Eol! Your father commands you. Leave the house of his enemies and the slayers of his kin, or be accursed!' But Maeglin answered nothing.".....

"So you forsake your father and his kin, ill-gotten son! Here shall you fail of all your hopes, and here may you yet die the sdame death as I!"

And how are free will and curses intertwined?

[The Saucepan Man]

And how are free will and curses intertwined?

Ah now, that one's a biggie.

As I recall, there is quite a good discussion of this issue in the context of the curse laid on Hurin and his family here:

Hurin and despair

Edit: I hasten to add that I am not suggesting that discussion of curses in the context of free will should not continue here. I am merely providing the link as it is relevant to the issue raised.