The allegory of the wedding party
One should know that the story of a bratty wedding party is an old, old trope, and it even shows up in the Bible, but in this case, the story is flipped. Instead of a wedding party who disrespects the groom, it's the wedding party discussing ways that the groom disrespected them and their family. Salinger uses this as an allegory for judgmental thinking.
The introduction of Seymour
By introducing the reader to Seymour, Buddy challenges the reader to interpret their own feelings about this fictional character, drawing attention to aspects of Seymour's life and consciousness that few people knew about, drawing attention to the religious philosophies of Zen Buddhism and Daoism. These are designed to be symbolic, because the reader might have opinions about the suicidally depressed, and Salinger is aggressively trying to reach through the book and challenge anyone who is judgmental about mental illness.
The sister's poem
The sister's poem on the bathroom mirror was a line from Sappho's poetry, and the full quote references a very tall, powerful husband who is coming for a wedding, like Seymour was supposed to. This is a symbol of the family's approval, showing that although Seymour may not feel able or willing to go through life's various stages, his brother and sister feel that he is already quite an amazing person, and they love him still.
Buddy's defense
Buddy is his brother's advocate, which says a lot about both brothers, because brothers could very easily become cut-throat enemies. But for Buddy sees Seymour as someone worthy of grace and charity, so he often argues for empathy and consideration. Buddy and his sister probably already know that Seymour is extremely depressed, and they don't judge him or hate him for that, as others do. All this serves as a symbol representing Salinger's own defense of the depressed and disturbed.
The motif of judgment or assessment
The backdrop to Buddy's defense of his brother comes in the first story when he is forced to listen to the wedding party's hateful language about his brother who failed to attend his own wedding. This is contrasted with Seymour's family who assesses Seymour very differently, and in the end, the Introduction is an opportunity for Buddy to put the question forward straight to the reader: "How will you judge my brother?"
