Regarding the Reveal Meal- Helen hadn't really planned anything out very carefully at all. I think we are missing a point, that would have been quite well done if it hadn't been overshadowed by the dramatic ending.
This is based on Helen's conversation with Kirsty at the beginning of the episode: Helen didn't think Rob was that bad, just that they were having a rough patch and it was because she was pregnant. She was planning on leaving for a while to get her head together and then thought they could rebuild things. She was planning on going to her parents after dinner. She thought she'd test out whether she was mad or not by making him tuna. She still expected them to have a fairly reasonable conversation "I need some space Rob" and him, whilst not liking it, would allow it. She never thought he would become violent (which he didn't) because in her head he was still RobHerHusbandWhoSheLoves and therefore would understand. She hadn't reached the point of really seeing him for who he was and being petrified of him. This is why she didn't take any precautions or leave whilst he wasn't there- she hadn't reached that level of understanding yet. She was just taking a break reform her husband who she hadn't been getting on that well with. I would imagine lots of women leave non-abusive partners in this way, even trying to be nice to them by talking to them and giving them a final nice meal together. And that for them, it goes fine.
It doesn't fill the plot holes of the half packed case and Henry being in bed. Nor why Kirsty wasn't sitting outside clutching the phone ready to answer to Helen/call 999 after she had heard Rob yelling and knew he was a violent abusive person.