Free Will and Turin Turambar

[Tuor of Gondolin]

1---I tried a search for this topic but I found nothing specific to the Curse of Morgoth on Turin and Nienor, so hopefully this topic isn't repetitive and is of interest.
2---If it interferes with or preempts the Silmarillion chapter project i imagine it could be shelved.
But, barring that:
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The ability of curses to occasionally seem to negate free will in Middle-earth (particularly in the First Age) seem to be an anomaly in a basic theme of JRRT's in his legendarium. While the Curse of Mandos was retributive justice for the Noldor's, especially Feanor's, kinslaying, in the case of Hurin, and even more Turin, Morgoth's Curse seems to make supposedly free wills hapless and helpless. Is there a rationale for this?
Paul H. Kocher, for one, in his 1980 book A Reader's Guide to The Silmarillion seems to have no answer.

In reflecting upon this grim tragedy of incest and suicide the reader is likely to ask sooner or later whether it is consistent with the doctrine underlying the whole of The Silmarillion, that Elves and Men have been created with wills free to choose between right and wrong.This is to ask whether Morgoth's curse upon Hurin and his children succeeded, and this in turn is to ask whether Morgoth or Iluvatar by his Providence governed the course of their lives.

We know the answers in general. Tolkien has been repeating them time and again from the Music of the Ainur on. In the foregoing tale, however, Iluvatar has permitted Glaurung to deceive and paralyze the will of Turin from the time midway through the story, when the Dragon first encounters him at the sack of Nargothrond, until Turin kills him just before the final catastrophe. And in his sister Nienor the paralysis goes so deep as to reduce her mentally to a little child, without memory, intelligence, or will. Clearly during these hypnotic trances neither brother nor sister is responsible for what is done.

(Kocher goes on to note Turin has free will before and after these events, but doesn't further address the seeming contradiction in the tale of free will/curses. Yes, Melkor was an extremely powerful Ainur, but is not free will a distinguishing feature of men, elves etc. from kelvar? And, also, why couldn't Melkor curse all his enemies and take over all Beleriand (except Doriath) with no trouble?