I was once told that we were all part of the same bunch, but I have grown to believe as you said that surely doesn't have to be true.It's apparent that the Cornish Kellows aren't necessarily connected to the Scot variants at all. The Cornish "Kellow" name may be more of an equivalent to common American names like "Woods" and "Groves" (I happen to have friends with each of those surnames over here!).
In America, there's a long-running theory that the "Kellows" are merely bad spellings of the "Killough" family, which is "Kellough" in some lines.That caused me a great deal of confusion over here for a long time until I kept finding a string of "Kellows" in America that I can now document link-by-link back to the State of Maryland in the late 1600's.That is a couple of decades before the "Killoughs" claim to have arrived here at all!
The Killoughs are quite convinced that they are of Scotch-Irish origin, and probably Scot originally.I have found quite a few records of "Kello" families in Mid-Lothian areas of Scotland back into the 1500's.There's almost no "Kellows" in those records though...maybe none at all.There's also a bunch of early "Kellowes" along the Scotland-England border.
At this point, I think the best guess I've heard is that the original Scottish MacKellochs and Kellochs gave rise to later Kello's in Scotland.Maybe some of these became Kellowes along the English border.When the Scots made their big move into Northern Ireland, some of the Kello's appear to have become Kelloughs and/or Killoughs.A group of Killoughs came to America in the early 1700's, mainly because of the political suffering and famine in Ireland at the time.Once branch of the Killoughs seemed to prfer the "Kellough" spelling.
A group of Kellows (my clan in Maryland) sneaked over a couple of decades earlier apparently due to religious persecution in Scotland (they may have been "Covenanters").
There's also a group of Kello's in America/Virginia that appear in the 1740-50's.They appear to be communicating with relatives in England by letters.I don't know if they were of Cornish origin or from further north.We also have some Cornish Kellows who came over in the 1800's I believe.There was apparently a lot less political or religious pressure on that bunch back in England and they may have come later simply for the opportunities available.
Overall, I think you're right and it appears that all the Kellows aren't necessarily connected at all.