Looks like you have been working on this brick wall for about a decade, so I doubt if I can tell you anything you don't already know, but I'll throw in my 2 cents worth.
David Pierpont is most likely a descendant of either John Pierpont or his brother Robert Pierpont.Both had immigrated from London to Mass circa 1640.
You mentioned that Richard Morgan was a descendant of John Morgan, brother to James and Miles."It is of interest that in 1640 John Pierpont married Thankful Stowe and moved to Roxbury, lately deserted by Miles Morgan, where Pierpont purchased three hundred acres of land and settled down to farm." http://archiver.rootsweb.com/th/read/GEN-MEDIEVAL/2000-07/0964245130http://archiver.rootsweb.com/th/read/GEN-MEDIEVAL/2000-07/0964245130
J.P. Morgan's mother was a descendant of John Pierpont.You can find more information about the Pierpont family in "Pierrepont Genealogies from Norman Times to 1913" By R. Burnham Moffat, which can be read or downloaded for free in Google Books. http://books.google.com/books?id=LIRIAAAAMAAJ&printsec=frontcoverhttp://books.google.com/books?id=LIRIAAAAMAAJ&printsec=frontcover