No, I am not related to this line.I descend from Jeremiah Murdock who was in Virginia in the 1720s, possibly earlier, and whose brother John was in Maryland by 1703 and possibly earlier.Family legend has that they were from Scotland.My branch moved to Kentucky, Ohio, Indiana, Illinois, Missouri, Tennessee and more.
That brings me to the Scottish naming pattern and I think it was practiced in a lot of families not just Scottish families and you may know about it.The first son is named after the father's father, the second son is named after the mother's father, the third son is named after the father.The first daughter is named after the mother's mother, the second daughter is named after the father's mother and the third daughter is named after the mother.So if Samuel was the oldest son and John was the third son, then Eliza's father was John.Further, John was the name of Eliza and John Steven's second son which according to the pattern should be named after Eliza's father.At least it gives you a name to look for.I would also bet that Eliza's mother's real name is Sarah since Sally is a nickname for Sarah.
Another thing to think about and why I am suggesting that you not rule out Eliza Clark as being the same person as Eliza Murdock.Even in current day, when one applies for a marriage license, a woman is always asked her maiden name.I have sometimes found the maiden name on the certificate rather than the married name - I don't have an answer for the reason that happens.It would have been unusual in that day and time for a woman not to get married until she was in her late twenties or early thirties.The reason Eliza Clark's daughter, Ellen, was not in the 1870 census with Eliza and John would have been she married prior to 1870.
As for the John Stevens/Stephens I found in the 1860 census.You can't rule him out either.It is quite possible he left hischildren with family while he settled in a new location.It would be difficult for a man to travel with young children while trying to settle in a new location.Maybe Patrick Burns was a family member.
Do keep in mind, there are many spellings for Murdock, Murdoch, Merdock, Mardack, Mardock, Murdick - just about any vowel variation you can come up with.
Oh well, it is just food for thought and something for you to explore.I do hope it helps.