Rail 2 for 1/Days out help

My family is coming to the UK in a few weeks. We are doing Edinburgh first then taking the train to York for 2 days and the train to London. I am wanting to take advantage of the rail 2 for 1 deals for some sites and want to know if I can use the train too from York for them? If I’m reading correctly, I can only use a coupon for arrival day as it’s only a 1-way fare. I have heard of dummy tickets that people buy to help cover the entire time there, would you recommend that option and just purchase a return ticket out of London and does it have to be from the same place you came from or where would you suggest? Or one of my sons and I are going to Harry Potter WB studios on our last day, would I be able to use those train tickets?