1. Wrong: Nan and Fedor couldn't hardly think about eating again after that huge meal.
Better: Nan and Fedor could hardly think about eating again after that huge meal.
Also better: Nan and Fedor couldn't think about eating again after that huge meal.
Explanation: The contraction couldn't and the word hardly are both negative and would therefore cancel each other out. The correct version can eliminate either of the negative words.
2. Wrong: Marquis did not have no strong feelings about the musical he saw with Manon.
Better: Marquis had no strong feelings about the musical he saw with Manon.
Also better: Marquis didn't have any strong feelings about the musical he saw with Manon.
Explanation: Both no and not are negatives, so they would cancel each other out. Either must be eliminated for the sentence to have meaning.
3. Wrong: Haruto didn't bring scarcely any warm clothes to Lakeville.
Better: Haruto didn't bring any warm clothes to Lakeville.
Also better: Haruto brought scarcely any warm clothes to Lakeville.
Explanation: The contraction didn't and the word scarcely are both negative. Either must go.