Well my goodness. Now, that may be the same person I found who said that the mother of Margaret Lockhart (daughter of John) who married in Lanark County, ON, was Jean McMillan -- which was what led me to that Elexis, the other daughter. The info about these people seems reliable.
So ... ruled out, it seems, which is what it's all about. The three unknown daughters, one of whom could have been your Ellen, have been identified and accounted for and ruled out.
The Lockharts John + wife + 3 daughters who immigrated to Ramsay Township, Lanark County, Ontario, in 1821 were not your Lockharts.
And while that may not be the answer one would have hoped for, it is progress!
(It turns out that there are a gazillion trees at Anc'y with the Thomas Cavers Sr. who married Margaret Lockhart, daughter of John, in Lanark County, Ontario, but no other Lockhart daughter info. Two trees say that the Cavers my acquaintance is descended from is a child of Thomas's second marriage -- which is false; his father was John, not Thomas. I gave up using trees at Anc'y as reliable sources a long time ago, after finding that my grx4 grfather who was a parish clerk in Cornwall, where he was hatched, matched and despatched in the 1700s, had a son born in Tennessee, and my Cheshire ancestor had a grandson 250 years older than himself ...)