First, a little background on American Quakerism.....the origins of the faithbegan here in 1683, with the creation of Pennsylvania, by Wm Penn, as a Quaker commonwealth.The vast majority of those who established the colony were English speaking folks....not Germans, who were almost all Lutherans or German Reform.The term Pennsylvania Dutch, had nothing to do with Holland, or the Dutchbut was Pennsylvania Deutsch, which sounded like Dutch, but was the word for German...so, they were folks who came to Pa from Germany...and, while there might have been a few German Quakers, there weren't entire groups of them, and nothing called Pennsylvania Dutch Quakers.So, I'd forget about the ancestors named Diehl, any spelling, being Quakers....much more likely to be the Copes...there were many, many named Cope, in Chester County, Pa, and the original emigrant, Oliver Cope, arrived very early (1681)...thus, you'll need to look to later Cope descendants of his for a relationship to Henry.
What is your source for Henry Cope's birth....Jan 15, 1790? (and, by the way, Quakers would never have used this sort of date...they had a very different way of dating things...no need to go into that now, but suffice to say, Jan 15, 1790 is not a Quaker date). Whereyou found the date migh be a clue as to know where to look.If you're familiar with the Quaker faith you'll know that Quakers were wonderful recordkeepers, and it's often possible to trace someone's moves all over the country, through their meeting records (Quakers don't attend church - they attend meeting), so births, marriages and deaths are recorded in the meeting attended by that family.
Do you know any of Henry's siblings?How about his wife, and names of children?That would help when looking through MM records, for the name , Henry Cope, isn't all that rare.I looked through some of the Ohio records, but don't see any sort of meeting in Marion County, Ohio.