Ah, an interesting question. Yes, he uses carnival in other ways. To make sure you've covered the setting the stage aspect, in carnival the whole world is turned upside down, and the unspeakable is done. The devil is freed, in a way, and that's what happens here.
Carnival is also used as motif throughout through the costumes worn. The mason "costume" worn by the narrator turns out to be literal; he walls someone in. By contrast, Fortunato's jester costume is made deeply ironic; rather than entertaining someone for fun, he dies.
As for the cask, there are several ways it is a very Gothic symbol of death. It is like a grave, and is stored in a catacomb. In casks a kind of transformation happens, in which juice is made into a more powerful and intoxicating drink. In the narrator's heart, Fortunato's many tiny insults are distilled into a poison that makes him drunk with the desire for revenge.