I think this is helpful because these are the same questions I'm asking myself. I've read the Gracy Whipple book and David L Pitcher is certainly not in there which means his line somehow got lost or, like we both originally thought, he is palatine.
One thing I think adds credibility to the "English" thought is the fact that David L. Pitcher gave his sons very "English" names: Royal (this is from Suffolk I believe), Lester (which is derived from the town of Leicaster) and George Washington (which, despite being truly American, is also English).
Not sure how much you know about this David Lester Pitcher but I have come to learn a lot. He was a 2nd Lt and Captain in the Civil War. I have been able to read many of his letters which are featured in an out of print book called "Full Measure of Devotion" by Willard Russel Dunn. As evidenced by the skill of his writing, David was an educator. He was a principal of a North Tonowanda school. He also was a member of the 'Sons of Temperance' which might be interesting if you've come across any other Pitchers with this involvement. My point in all of this, is that it would be hard for me to believe... given the amount of letters he wrote to the Lockport Journal & Niagara review and his well-documented and somewhat celebrated (wounded at Cold Harbor) Civil War experience... that Grace Whipple wouldn't of been able to make a connection.
I share your frusteration as we can find equal evidence to why this David L Pitcher could be either German or English. He did marry a woman by the name of Louisa Brewer whose influence might've been a factor in giving his children such English names.
The Peter Pitcher, Peter Pitcher Jr., John Peter Pitcher, David Pither connection is just too hard to ignore despite my inability to make a direct connection. I think somehow these people are related...
May I ask what and where you fall in the Pitcher Line?