I haven't read or seen the play before watching Whedon's adaptation, although a brief bit of research afterwards uncovered that the text does provide some support for the opening scene in which we see Benedick and Beatrice wake up after a night together. The conversation which hints at their estrangement is included in the film ("you have lost the heart of Segnior Benedick"), but I didn't clock it when I watched it. It's a smart addition not only because it explains the bitterness between the two at the start, but also because it links in with the broader themes of the adaptation. Whedon is open about the sexy places he wanted to take the story, and switching Don John's follower Conrade into a female lover was done purposefully. The license displayed at the parties in the mansion serves to highlight the hypocrisy underlying the accusations levelled at Hero at her wedding. Male pride demands that the wives men choose for themselves remain chaste, and yet these rules are frequently (inevitably) flouted, hence the obsessive talk about cuckoldry. Moreover, accusations of impropriety are weapons used to inflict harm on political enemies, women being used as discardable tools in the struggle between male lords.
If there is one possible misstep in the whole film, it is in the portrayal of the grossest example of this behaviour. Margaret is seduced by Borachio, and the way the scene is shot suggests that he might be raping her. Later on, Whedon includes Margaret's jab at Benedick "Give us the swords; we have bucklers of our own", which echoes Beatrice's speech at the film's climax, and it's a quiet moment rather than a funny one. But the weddings at the end brush past Margaret's experience a bit too swiftly – perhaps Whedon could have lingered just a bit longer on Ashley Johnson before the party starts swinging again at the end.