I doubt if this is a direct use of the Welsh name. It is rather probably a transfer from the surname.
Emlen or Emlin is the surname of a family that was prominent in the early days of the Quaker settlement in Pennsylvania. George Emlen was a settler in Philadelphia from Somersetshire in England. It seems that most Americans with the surname Emlen or Emlin are related to him.
On Ancestry Library I was able to find from records of the 1855 state census of Iowa that the parents of Emlin McClain, the Iowa judge you reference, were born in Pennsylvania. So that would seem to provide a geographic link to where most people with the surname Emlin lived.
According to Reaney & Wilson's A Dictionary of English Surnames, as a family name Emlyn goes back to the Old French female names Ameline or Emmeline, not the Welsh name. But I don't know if anyone has traced that derivation for sure for the Somerset family. Since Somersetshire is just across the Bristol Channel from Wales, and many early Quakers in Pennsylvania had Welsh ancestry, I suppose there is an outside chance that the American Emlin family derives there surname from the Welsh Emlyn rather than Emmeline. But that would require someone who's an expert on English and Welsh genealogy to answer for sure. :)