3.1 gives us more exposition from Salerio and Solanio, and then a big moment for Shylock. Again we get two sides: his calls for “revenge” are ugly, but he also gets a big speech: “Hath not a Jew eyes?. . .” that has to be designed as a series of questions to the audience as well? Interesting that he begins with “eyes,” since we have already been invited to think that what the eye sees (in the caskets) is misleading. It’s odd that Shylock assigns such value to the plain ring Leah gave him, and by contrast would rather have his ducats back than his daughter. It’s shocking that he “would my daughter were dead at my foot.”
3.2 is the longest scene so far, and seems (as you suggest) to conclude the “marriage plot.” It seems to me that the fix has always been in. Both Nerissa and Portia are rooting for Bassanio, and of course he is the winner. It’s interesting that Portia seems to be the more aggressive wooer — maybe we are to assume that the usual love-wooing has already taken place off-stage. (That’s interesting in itself.) Bassanio says a lot about gold, but his dismissal of silver and his choice of lead seem perfunctory — as if Shakespeare is impatient to move on. Portia respond with a lot of language about accounting. As you say, she is disingenuous. When she commits everything to Bassanio, she says what’s mine is all yours. Maybe what’s important is that she makes free unconditional gifts, unlike Shylock who makes sure to secure any loan he makes with a “bond.” But later the ring she gives Bassanio will turn out to be a kind of bond. She also says that what’s Bassanio’s is “half” hers. It’s now that he tells her that he has no money of his own — he presumably spent all of Shylock’s money to make himself look flush. All she gets from Bassanio is a share of the bad news about Antonio’s losses.
3.3 Meanwhile, Shylock has upped the ante: he won’t take Portia’s money and thus let Antonio off the hook. And we also find out that Shylock has more motive: Antonio has several times “delivered” Shylock’s debtors by bailing them out.
3.4 In this scene we hear that Antonio is the “bosom lover” of Bassanio. Closer and closer.
3.5. Another scene with Launcelot. Maybe, as you say, Shakespeare had to write something for the actor. I’m tempted to skim. But can we find something in his lines that advances or complicates the action? A lot of verbal “quibbles” in this scene — and elsewhere. Johnson thought Shakespeare couldn’t resist them. Are they doing some other kind of work?
One backward glance at the casket choice, Aragon’s choice of “desert”: in “Hamlet,” coming in a few years, Hamlet will memorably say to Polonius, in response to P’s saying he will accommodate the players according to their desert, “God’s bodkin, man, much better. Use every man according to his desert and who shall ‘scape whipping.” Rather a consistent thought on Shakespeare’s part. I would guess the answer to why Bassanio “deserves” Portia is that he doesn’t, and no one could. His getting the right casket is a matter of correct understanding of human nature, rather than moral worth, so already we’re also in a play of ideas. I think Bassanio has to be played carefully.
But I agree that Bassanio seems a somewhat pale lover. He does have some strong poetry, and is unfailingly loyal to Antonio. But he is perhaps something of a cog, which may be why he’s assigned the strong poetry. He can’t really be worthy of Portia, but who could?