Yes, Cathy would have insight into being hoodwinked by someone who seemed nice but turned out to be an evil abuser - which ought to get Pat thinking about whether Helen was raped.
I can understand that Helen's mind would go into shutdown and everything seem exhausting and circular.
I do not understand Pat's two week+ ineffectual flap. Were I her, I'd have gone into stress-driven obsessive mode, slept little, drunk a lot of coffee and written long lists which I'd have turned into a 'signs things were not right' spreadsheet. I would also have demanded that Kirsty contribute; sat her down and fed her biscuits and the occasional sandwich until every instance of Helen's odd behaviour, worrying comments and Rob's unpleasantness had been noted and dated. Same for the rest of the family.
Then I'd call on every acquaintance of Helen's, talk things through (probs record our discussion) and ask them to fill in their own column / draft it for them and ask them to check and amend.
As well as gathering evidence, this would serve the purpose of finding out who was 'with us' on this matter and getting word out about how Helen had been abused and it was not her fault.
They'd all think I was overwrought and a pain in the bum and it would probably drive them towards requesting to make a statement to the police, so they could tell me they'd done it and I'd go away but it would get them thinking.