I'm not sure that the ongoing explanations that original Peter died will work anymore. Granted, the gravestone adds some confusion. The observer is asking Walter how to stop a death from happening. It seems to me that Walter did the same thing. He did something to placate fate, which wouldn't be necessary if fate had already landed it's blow.
Would he really lose a son and then put other world Peter in additional possible harm?. That just doesn't make sense, even given Walter's need to have his son back. And what of other world Walter. Why would he be so passive in letting this world Walter take his son?
What makes more sense to me is that Walter's original Peter became very sick and was somehow marked for death and Walter somehow figured out that no matter what he tried to do to save Peter, the observers would counter that. Finally, to make Peter important, Walter switches his son with the Peter in the other dimension. Perhaps this is done with the Walter in the other dimension, whose son is also about to die.
Maybe the gravestone is more a memorial to the son that he will no longer see than an actual grave. Or maybe not.
