We do not have any examples of Morgoth putting the Spell of Bottomless Dread on a human, so I'm not entirely sure why Húrin is part of this conversation.
Well, you appear to be saying that Morgoth can automatically put it on anyone no matter what, that he succeeds every single time he tries, and that the only reason he doesn't do it to everybody he meets is that not all prisoners are important enough. I don't know why any automatic "Erase Free-Will" button would work on Elves but not short-lived, weak-willed, weak-spirited Mortals. So I'm trying to say that if it's automatic, then Morgoth must have deliberately avoided breaking Hurin because he was a less important prisoner than Edhellos, and Gondolin wasn't important to Morgoth.
Either Morgoth wants to break Hurin, tries, and fails,
or he does
not want to break Hurin at all, and doesn't even try.
We should show the first, not the second. Morgoth by that time believed the Valar had abandoned Middle-earth forever, and that Turgon was
the greatest threat to him. The Spell of Bottomless Dread can't be as utterly irresistable and 100% automatic against everybody as the lure of the Ring. It can't be an "I win, no saving throw!" button. We may not have any reason to show somebody resisting it, but we should write consistently with the idea that Morgoth can't automatically do this to every incarnate ever.
I also don't see what's so wrong with the explicit statement by Tolkien that it was flat-out impossible to put the Spell of Bottomless Dread on most Noldor, and that most Edain were also impossible to break. Nobody else has even acknowledged that Tolkien wrote that.
If Morgoth finally let him go, it was because he thought Húrin was "ready".
I'm pretty certain that he ran out of ways to break Hurin, and that was his last shot. It also almost failed, because one of the earliest things Hurin did was contemplate suicide, and the Eagles were about to pick him up and coincidentally kill the spies.