ooooh. I have just come up with something.
So I just took 2 paracetamols IN CAPSULES. I've had zopiclone before, they're pretty small so I just had the thought when seeing the capsule size that you could probably get a decent dose of zopiclone into those capsules.
ANYWAY. What if he couldn't set a day for the murder, WHAT IF, he has to wait till she has a headache and ASKS HIM to bring her some painkillers. This would explain why it seems he didn't plan it for this day and it's all done short notice, e.g cancelling the docs at the last min. It would also explain why she conked out when his parents were there, she might not have fallen asleep had she not had a headache!
there was paracetamol found in her system as well remember.....