semicolons are used to separate two independent clauses. Independent clauses are phrases that make sense on their own. The comma is needed in #2 because the phrases don't make sense on their own. In that case, a semicolon wouldn't work there.
Mood isn't a literary element in a story, a novel, or even a book. Mood is what the author feels and what changes in the book. Like from being sad to being happy.
