What are the implications of Holling Hoodhood's decision-making, and how does he change throughout the book?

Regarding your first question as to the implications of Holling's decisions, we need to start with a decision that has some major impacts on the story, his growth, or his relationship with other characters. If I had to pick a single decision that Holling makes that I feel shows the greatest strength of character and growth in Holling, it would be his decision to rescue his stranded sister, Heather. For most of the novel, Heather is quite antagonistic to everybody in her family. Her dad angers her a lot, but Holling is generally the recipient of her wrath. In Heather's defense, Mr. Hoodhood is not a loving father, and Holling is very intimidated by the man. When Heather runs away, Mr. Hoodhood expresses little concern, and he's not motivated to help her once she becomes stranded. It's Holling that goes against his father's example and rescues Heather. Holling puts himself out on the line for his sister's sake, and it pays off in their relationship. Readers finally see Heather's icy exterior break, and we see her transition into a loving sibling. We know that she will be there for Holling if and when he needs it. His decision to rescue Heather has huge future implications for their relationship. No matter how bad a family situation their father creates, Heather and Holling will be there for each other.