Overall, how "comic" is the Merchant of Venice? Does the final act succeed in restoring comedy to the play?
Great question. The answer to the first question relies heavily on the play's audience. Shakespeare's audience would have found the play more humorous than a modern American audience. His audience, because of England's (and Europe's) long history of prejudice, would have laughed at Shylock's fuming over losing his jewels and daughter and perhaps even over his forced conversion to Christianity in Act 4. Additionally, the ethnic slurs against Shylock made by Antonio, Gratiano, and even the Duke would have drawn laughs.
In contrast, most Americans would not find Shylock's downfall very funny. They are appalled that a court would force someone to give up his or her religion in exchange for another, and most of my students and others find Antonio's character manipulative and just as questionable as Shylock. For example, in Act 4, Scene 1, the Duke allows Antonio to present an alternative sentence for Shylock, and Antonio designs a sentence that benefits himself. He does not have to pay Shylock back; he is entrusted with Shylock's money for Jessica and Lorenzo, and he no longer has to worry about Shylock practicing usury because he includes Shylock's "conversion" in his alternate sentence.
All audience should find comic relief in Lancelet and his father Gobbo. Their use of malapropisms in Act 2, Scene 2 is comical, and Lancelet continues to lighten the play's mood later in the play. Likewise, Gratiano's nonsensical speeches and double entendres are meant to cause laughter, and Portia's description of her suitors in Act 1, Scene 2 are funny even to a modern audience.
In the sense of a Shakespearean comedy, Merchant does fit the general requirements. The play features marriages, a clownlike character, witty verbal jousts, and a happy ending. However, it also demonstrates Shakespeare's maturity as a playwright. He uses the comedy genre to deal with Elizabethan Era issues--usury and discrimination. While Shylock's character does present a problem with the comedy classification because he possesses many of the attributes of a tragic hero, overall the play, especially because of Act 5's resolution of the "ring plot," is a comedy.
Further to the earlier answer, most of which I agree with, I would say that the final act lets the play down badly rather than rescues it. Whilst I agree that Elizabethan audiences might have reacted differently from modern ones to Shylock's downfall, I still find it unpleasant to have the high drama and emotion of the trial followed by these mostly unattractive, frivolous characters, high on their 'victory', playing silly games full of really crude sexual innuendo. I especially detest the very weak last line, which may have had them rolling in the pit circa 1600, but for me just about sums up the messiness of this problem play.