Did Cersei really plan to have Robert killed in the confusion of the melee? We never really hear anything about it during her POV nor does she ever mention it during conversation with any other Lannisters. Did Varys invent it to fuel the Stark-Lannister friction & prove his loyalty to Ned?

1. Yes. We see Cersei using reverse psychology to get Robert to sign up for the melee ahead of time. 

2. Why would Cersei spend any time talking or thinking about a failed assassination attempt after the successful attempt went off?

3. The two assassination attempts bear a marked resemblance – they both use Lannister loyalists, they’re both “accidents,” and they’re both really shoddily relying on random chance to actually kill Robert.