It ends when the main conflict is resolved and all loose ends are tied up. That's just how most stories work.
It usually ends when the main conflict is resolved and the characters' arcs reach a satisfying conclusion.
It's hard to say exactly. Maybe in a particular context or story, 30 has been assigned a symbolic meaning representing the conclusion.
Maybe it's to create a big shock and leave a lasting impression on the readers.
Maybe it's to create a sense of mystery and heightened tension at that specific time.
Else might smile at the end because of a hidden surprise or a realization that made her feel good. Maybe a long-awaited event happened, or she overcame a big challenge.
Maybe Fortunato laughs out of shock or a sense of irony. It might also be a sign of his inability to fully comprehend the situation he's in.
Maybe he's just relieved and happy that everything turned out okay in the end.
Maybe she's sad because something unexpected happened that broke her heart.
It could be due to his mental state and the stress of the events. Lennie's mind was already fragile, and the culmination of everything might have pushed him over the edge, causing hallucinations.
Maybe he's happy because something good happened to him. Or it could be that he finally achieved a goal he was working towards.