I would not necessarily characterize B.R. Howard as an "Artzy fartzy museum restorer not affiliated with railway preservation." Before going into private practice about twenty years ago, Brian Howard worked for many years as the head conservator for the Pennsylvania Historical and Museum Commission and did some work for the RR Museum of PA on various artifact projects and is a skilled conservator of historic transportation vehicles. One of his associates just did the gold leaf work on the PRR No. 331 on the Harrisburg, Lincoln & Lancaster RR. He has also done conservation of stationary steam engines such as the 1851 West Point Foundry engine at the Cornwall Iron Furnace recently.

Seems like the only way to settle who built Yankee is a metal analyses of several parts (boiler, frame, cylinders). That should at least tell if it is English or American or both.