Martin wouldn't have told them things if he wasn't okay with them using the informaton.
I doubt he seriously considered the possibility one way or the other. He was giving them information to help with long-term planning, not a way to replace him.
They don't have to know every detail, as long as they know what happens to the major characters and plotlines it's not hard to figure out a way to get them there. They are changing parts of the story anyway, so it doesn't make much of a difference.
I think you're overestimating what they know, and underestimating the complexity of the narrative. Sure, they could slap something together, but would it be satisfying as a resolution? If it comes down to "make something up or you're canceled," they might take a stab anyway. But I don't think that's going to happen.
The Arya/Tywin scenes are superficially well-crafted and amusing, but repetitive, and they undermine or displace the themes those characters are meant to reflect. Benioff and Weiss seem to think that Martin's less pleasant characters (Tywin, Cersei) need to be made "ambiguous" or "nuanced" by giving them heartwarming parental moments to balance out the nastiness; I don't find it at all convincing.