How is marriage represented in Henrik Ibsen’s A Doll’s House? What evidence can you find to support the interpretation that this play is not only about the Helmers’ marriage but also about the institution of marriage itself?

At the start of A Doll's House there is no real indication that anything is wrong or unusual about the Helmers' marriage. This in itself does not prove that it's "typical" or representative of the institution of marriage overall. But any theatergoer or reader of Ibsen's time, and later, would recognize that the dynamic between Nora and Torvald is normal in an age when few middle-class women had yet entered the work force and were therefore completely dependent on their husbands for money and basic necessities. The strongest evidence that this marriage isn't unique, and therefore is representative, is evidence by default: nothing shows it to be unusual.
All indications are also that Nora loves Torvald, and in fact the one mistake for which she's in trouble—forging her father's signature on a loan application—was done in order to obtain money for Torvald's medical treatment. As the play progresses, one sees more and more that the dynamic between them is dysfunctional, but this is so gradually and subtly shown that, again, we have no reason to believe this isn't a "normal" marriage.
It's really only with Torvald's ugly outburst, upon reading the blackmail letter from Krogstad, that we see something that might (even this is questionable) be too abusive even for most men of a time when it was normal to treat one's wife in a domineering way. Nora is suddenly awakened, as it were, from a dream in which she believed her domestic situation was happy and ideal. Yet, how many millions of wives have had such awakenings over the centuries?