Unfortunatly, the only way to know for sure (about the duke and dutchess thing) is to have been there, and that's not going to happen.It is possible that this person paid quite a fortune to contact British Genealogical authorites and gotten this info.England and it's Isles have kept accurate records since the beginning of time. Some of them ofcourse, have to have gotten lost, or destroyed through time.But I still wouldnt put any weight on what that person said.
The only pictures I currently have of Lt. Gov. Paul Brigham is a sketch copy of the portrait that hangs in the Bennington VT museum that I got out of my History of the Brigham Family book.
I am his 4th great grandaughter from his daughter Lydia who married Joseph Lathrop.Sounds to me like you have more information that I do. I dont remember my book mentioning any specific Royal connection. But I could look again.