Ophelia's madness shows us the damaging effect of all the shenanigans and double-dealings that have been going on at Elsinore. Ophelia is a sweet, innocent young lady shamelessly used and abused by her brother, her father, Claudius, and, of course, Hamlet himself. She's one of the few recognizably normal characters in the play, in that she isn't morally compromised in some way, isn't involved—at least not by choice—in any shadowy court intrigue.

In scene 7, Claudius is thinking only of himself, as usual. He gets Laertes involved in a plot to finish off Hamlet once and for all in order to protect his throne. As for Laertes himself, his sister's death brings his impetuosity to the fore, and he immediately dashes off to gain revenge on Hamlet, thus jeopardizing Claudius's carefully laid plans. As for Gertrude, she appears genuinely concerned over Ophelia's tragic death, but at the same time seems to be in the dark concerning the bigger picture, as it were, or the reasons for her suicide. In other words, Gertrude can be read as innocent and naive in her own way.

At least in part, Ophelia's madness is there to show true madness. Just in case anyone should think that Hamlet is mad, Shakespeare presents us with a person who is really bonkers.

Like Hamlet, she has lost her father, but unlike Hamlet, she has also lost her wits. She acts strangely, is distracted, sings, then talks, cries, seems deeply troubled, and she can't stop the things that are botherering her mind from spilling out of her mouth. And, in the end, she ends what has become of her pathetic existence.

Laertes is presented in Act 4 to show us what revenge looks like. Without question or thought, he is ready to slit Hamlet's throat.

Yes, in many ways Ophelia and Laertes are a pair of characters that shine a light on one character: Hamlet. They are two extreme reactions to a father's murder that, in Hamlet, are co-mingled. Hamlet does his best to assimilate both his mad rage and his need to exact revenge in his own way.

Now I'm sure someone else will graciously give you an answer to the rest of your question.