I'm curious what she hoped to get out of lying about Kovu being in on it. We know that Kovu was exiled because Zira had said he was part of it, but obviously Zira DIDN'T intend for Simba to get away alive from that.
So, I'm curious, what would it have mattered if she had said "We're here, no thanks to Kovu" and thus showing that Kovu HAD indeed double-crossed her. Actually, given how Zira had later told Vitani that if she wouldn't fight that she'd be killed too, I'm surprised that she didn't order her pride to off Kovu along with Simba, nor why she didn't try and kill Kovu after Simba got away and Nuka was killed.
Was she hoping that by trying to implicate Kovu in this, that he'd know that he couldn't go back even if Simba survived, and hoped that would motivate him to return to her side?