First you are right, that Philip was born to Johann Jacob Jan. 17, 1789 in what was know as Germantown PA.
I"m not at my computer so I can't give exact dates of the move, but I don't think there was one exactly.There is a will for Johann Jacob and I do suggest that you order it from VA. Since I think you would need it anyway for any application.
If you check the census records in PA. you will find that there is another Peter Cosner living in Bucks County, Plumstead Township.He continues to live in the area, even after our Philip Peter moves to VA. (I've never found any records of our Philip Peter going by the name of Peter.it was always Philip. I suggest you start there and you will find that the military information pertains to him.There are many holes in the DAR application that was approved many years ago.I doubt that under todays scrutiny that application would get approved.
Also, Mary Cosner married Joseph Johnson Nov 6 1854 in Knox County OH.'
If you haven't research this family, then you do have your work cut out for you.I would start with the census records as they will help the timeline.