The theory that Simon was her target, allegedly, if she is guilty, still works, whether it was before or after he put his status as separated. IMOOh no, I've just wasted so much time!
That is my theory too (allegedly, if she is guilty).
I think it explains her anger that he had put down 'separated' before she had fulfilled her (alleged) plan, because it would show a motive that he had no intention of reconciling, if he died after that. Hence a plan (allegedly) that would be more likely to be accepted as an accident if it wasn't just Simon who would die, because she would be lacking a motive to poison everyone.
I go back to her interview with press outside her house and her leap straight to reasons why she wouldn't kill them, I think exploiting the opportunity to have a platform, but having no intention of answering anything she was asked -
"who are some of the best people that I've ever met"
"Gail was the Mum that I didn't have"
"Gail's never been anything but good and kind to me"
"Ian and Heather were some of the best people that I've ever met, they never did anything wrong to me"
"I'm devastated, I loved them"
"[and the loss to] my own children who've lost their grandmother"
IMO, just a theory, if she is guilty.