The main conflict is that Harry is now old enough to face voldemort for the first time. however harry does not know this so he assumes that this fight will be between him and snake but it will actually be between him and quirrel.
5 is opinion so you can basically say whatever you want
A novel can have loads and loads of conflicts big and small, while a short story has only one conflict. The characters in a novel are usually dynamic and are better described, while characters in short stories are poorly described and are usually static.
I would say it’s the first or second. I’m leaning towards the second. Mostly because I don’t know key ideas from this story.
When asking about books, I suggest using shmoop.com because it gives short but descriptive chapter-by-chapter summaries.
