Would you be fooled by someone pretending to be your father? And for what reason would he have to kill two small sleeping children? If he had a beef with Nevill, I very much doubt he would wait until Sheila and her children were there. He sounds like the kind of person who is reactionary and does things on the spur - not sneak into someone's house in the dead of night.
I happen to have an extreme memory for voices, so no, I wouldn´t be fooled. I don´t know how Jeremy´s voice recognition capacity is. He claims he was woken up, probably confused, conversation very short.
I don´t seriously think this happened, but nothing is impossible, and I am always looking for that missing piece. To me the case is not solved.