I think she thought they'd all be dead before they were, and that all the traces of DC in their bodies would be gone, and they would not suspect DC poisoning, let alone treat them for it, and have one guest survive.
She didn't plan for any of that, and started deflecting, blocking, and spinning more and more outlandish stories. I think she's been reading Websleuths during the trial, also, because she's borrowed answers from them - the coffee that no-one with diarrhea would drink, that he son says she was drinking the morning after the lunch, is now a herbal tea, etc. Possibly ginger and lemon!
But she can't remember her previous lies, even ones told the same day, and keeps contradicting her own evidence - as well as claiming everyone else is either lying or misremembering, including medical staff who have notes to refer to, and her own children, and of course the survivor, Ian Wilkinson.
In short, she's nuts. And very arrogant and cold, and also lacking in any empathy for her victims, which you would think she would at least try to fake.