I am traveling today and just read your post! Something to ponder does not begin to describe what we have. Unfortunately I will have to wait until Sunday to delve back into the thick of it.In the meantime, know that I cannot wait to dig further into my files and see if anything else aligns with what you posted today.
Most of my records were obtained the old-fashioned way, by traveling to Westmoreland and Armstrong Counties. While I do not have Ancestry.com, this should convince me to take the leap. I do check other sources online, and will continue to do so if there is even a small chance that I can find another Pettigrew tidbit.
One of the Armstrong County Census records had a Joseph Pettigrew and family living close to Jacob. I cannot remember the year at the moment. When I wrote another participant re: this Joseph, her response was that Joseph and his family moved back to Allegheny County and she did not feel they were connected. I do, however, given their proximity.
Your point re: John possibly passing away and Jacob living with Robert the "uncle" makes sense. Robert had a son by the name of Joseph, and perhaps that was the one who located near him.
As for the "Signers of the 1784 Petition" ........Fantastic! Petticrew and Pettigrew in Dauphin County. Imagine. I have never been to Dauphin County to do research, have you? This might be the summer to make the trek.
I take my leave for the moment, but will be thinking of the Pettigrews/crews to be sure!