This is where I struggle with her going ahead after Simon didn't attend.
She had the intellect to formulate the plan, but it clearly revolved around them all being dead, with no certainty about what happened. If she was queried, she had answers prepared to at least create doubt. Best case, it was never even linked back to her.
Once Simon didn't come, she had to be aware it would come down to a "he said, she said" scenario, where the ability to play "the appropriate role" was going to be necessary. Was she really just going to rely on disposal of evidence and bare-faced lies?
With what we know now, Simon was going to immediately suspect what happened and cause her serious issues. Was she just planning to torment him for the rest of his life, with him knowing, but unable to prove it? Relying on cold, hard facts (her version thereof) and not realising her cold, hard demeanour would be a strong indicator of guilt?
You have to wonder whether an unfortunate accident may have befallen Simon in the days following the lunch, if the others had all quietly deteriorated without revealing anything to authorities. Otherwise, what the hell was she thinking in terms of how it would play out?