Would Hel have kept her bargain with Thrym if he succeeded?

[Kardwill]

Did neglecting the demigods lead to the current state of the world?

Nope, the gods and mortals didn't need any help to screw that part themselves. But neglecting the demigods led some of them to vote out of spite or because it was their only hope to get their voice heard at least once. Everyone was surprised so many of them were even at the moot, and they had to look for them because they didn't know where they waited. The demigods were nobodies, and so were completely unpredictable since none of the gods apart from Hel bothered to figure out what would happen nor seek their favors.