I decided to dive into this thread just a little bit ago, had no idea what I was getting into!
I realize we don't have all the evidence, of course, but I'm definitely finding EP and her actions highly suspicious.
I also find it interesting that so many people seem willing to give her the benefit of the doubt, whereas I think if she was a man accused of attempting this sort of thing, I don't think that would be the case. And they do say poison is a woman's weapon.
As for motive, I do believe it's likely financial, custodial, and revenge. I surmise that if Simon and her were only separated, perhaps he was asking for a divorce, so he could move on to the next relationship. A divorce would be final, and if she is the one who had more assets, she would have to share them with him potentially (I do not know Australian law on this though, but in the US, it varies from state to state, but there could definitely be a situation where he would be entitled to 50% of all her assets) and perhaps a different custody arrangement.
It could also be revenge. "You left me, if I can't have you no one will". And then when he didn't show up, she decided to inflict pain on him in a different way.
There was a recent case in the US where the husband shot the kids right in front of the mother because she was going to leave him. He didn't kill her, but he did kill the children knowing the pain it would cause her for the rest of her life.
This is an unusual situation, no doubt, because it involves the in-laws. But if she saw them as influencing him to not get back together with her, then that definitely could be motive. And anyway trying to ascribe rational motives to murderers, is always almost an exercise in futility.
Have they said how long toxicology is expected to take?