Regarding the Earwood/Yearwood confusion, I tend to agree the confusion begins with those recording in written form the name as spoken. From my own experience, as I have related to others previously, my father would often become frustrated with clerks who would attempt to spell his name "Earwood" after he thought he had clearly said "Yearwood." There was, I assume, something about the way he sounded the letters which created the confusion. He was Southern born, Arkansas, and Southern raised, but living in what was then a new melting pot, California. Perhaps more clerks were about in those days with less use of cards and printed IDs, I have not experienced the same problem to that degree. Rather when asked to spell the surname I get the response, "just like it sounds."
Also, I have been told a story, second-hand several times over, regarding the Earwood spelling in which that spelling was adopted by one family line after a pension check was so issued. Unfortunately, I have yet to do follow-up research on either this story or line.
While you cannot claim that in such a case where a misspelling would lead to a plausible surname, all lines with that surname must come from a common ancestor; I feel confident that until you determine that a particular line has other origins--such as where a surname is simply adopted--we will be well guided by researching all the possible spellings or variations as if of a common root.