Trying to reconstruct a McLain family tree before the 1860s is pretty impossible. Not only are the records pre-1860 not easy to locate but this task is made even more difficult by the fact that every McLain descendent of the first guy who made it to the USofA wanted to name one of their kids after the patriarch.
His name was Fulton James McLain and he was born in County Antrim, Northern Ireland in 1770. He boated himself over the new world in the early 1800s, had a few kids and of course named the boy Fulton. He in turn named one of his sons Fulton and so on and so on.
In short this

may be completely wrong. Maybe Henry didn’t even know that Maray Helena was his cousin. Maybe he just avoided anyone with the name Fulton in an attempt to not inbreed. Who knows?
On the upside I finally figured out who this is.

This is, wait for it… another Fulton McLain. I believe this is actually my (Sylvia’s) great great grandfather Fulton James McLain III, but I’ll keep you posted…