I didn't understand how the gravestone is supposed to make their deaths a fixed point in time, it's just a stone, how does it prove anything? For all we know Amy and Rory buried their poodle and wrote their own names on the grave because they saw the stone in the future.
It was a big mistake the show showed the Doctor dieing and kept yapping about how his death is a fixed point in time only to reveal that only the damn robot being shot and the Doctor giggling and waving inside it was the fixed point. How is the audience supposed to take the fixed point in time business serious if there's always a way to weasel your way out of it?
I'm not even sure what the big deal was at the end, the Winter Quay was destroyed and most angels killed by the paradox, right? The remaining angel couldn't send Rory and Amy to the creepy room of dieing alone, it had to use the regular method to dump them somewhere in the past where they just live their lives. River even meets them and gets Amy to write the afterword. If the Doctor can't go to New York Amy should have just written "Afterword: Dear Doctor, pick us up in Kansas City on April 4th, 1942. Bring a gravestone. Thanks. XOXO Amy and Rory!"