I have no issue with seeing Ursula as a horrible mother to Rob; it doesn't excuse his behaviour. It might explain it but it doesn't change anything. And we know anyway that abuse is a repeating pattern, so it's not a big surprise.
My guess is that she will keep up the mocking and belittling remarks to Rob throughout the week and he will explode once she's gone, furious at Helen for what he will perceive as colluding to undermine him. The belittling will show him up as an inadequate, pathetic figure, and (as we know already) full of dark rage and obsessed with trying to prove to the world, and presumably to himself, how clever and charming and all-round perfect he actually is.
I also don't think that Rob has a grand, detailed, machiavellian plan to steal the business/inheritance/money, or to have Helen sectioned, or to have Henry taken away, or any of those things. All of it is about an overarching sense of control; he wants to show that he's in charge - he makes the decisions and the people around him tell him how great he is. That's his overriding motive, I think - to feel strong and powerful. As he obviously lacks insight or emotional intelligence, it hasn't occurred to him that manipulating vulnerable woman and small children, and lying to everyone else to get what he wants, are not the hallmarks of a strong and powerful character.