When I want to find out how to do something like this, I figure out who would do it all the time, and ask them. In this case, I would call companies that do cleanup and restoration from fires, like ServiceMaster. They should have some good suggestions for you, because THAT'S WHAT THEY DO.
Other than that, I don't know. You say you have tried 35 different things, but I don't know what they are. I would have to suggest 36 different products in hopes of suggesting one you haven't used.
I'm not going to do that, because I can't event think of one, short of just scrubbing the surface of the sandstone until you get down to a fresh surface.