The Mystery of Nannie Ferguson

I think I have caught the family history bug. Unfortunately, with that bug, does not come a knowledge of how to deep family history research.
I have been researching the history of my family and their relationship to the Mormon Church, and have found interesting stories I have never heard before. Many family stories come with an oral history, and change from generation to generation. A few decades ago a family member recorded many of our family stories and published them as a collegiate research paper. I've used her book as my primary form of research.
I noticed throughout her book (written in the 1980's) that when there was a discrepency among the oral histories that she recorded each of them. During my review of her work I realized I could probably clarify some of the discrepancies with some of the more recent technological developments. Sure enough, within a minute of Googling, I was able to figure out the real names of a few people. One of those names was Ammon Mercer, the missionary that baptized many of the earliest members of my family.
Originally I was searching for more information about Ammon Mercer, in hopes of maybe finding a missionary journal that might shed some light on a few of the other open-ended questions. During my search I stumbled on something interesting- he had more than one wife! He was most definitely a polygamist. And one of his wives was Nannie Ferguson.
Why do I care?
Well, the family history I am researching is the Ferguson family!
Who is this Nannie Ferguson? And why have I never heard of her before?
The man who baptized my earliest Mormon ancestors married a Ferguson, and somehow this detail has never been mentioned before!
I am intrigued.
And I have gone on a search for more information on this mysterious Nannie. But I am thwarted at almost every turn.
Here is what I know-
She was born near Roanoke where my line of Fergusons hail from. This strongly implies she is related, but Ferguson isn't an uncommon name.
I have found her father's name, and her mother's name. her mother is a Poage. The Poage family are from right here in good old Back Creek. In fact, there are still a few roads and buildings named after them. Again, strongly implies they are from Back Creek, and therefore related.
Her siblings include multiple Williams and Henry's. Significance? It is a Ferguson family tradition (still going strong today) to name a son William and a second name with an H, particularly Henry. There is a long list of William Henry Fergusons. A very long list. Of all the evidence, this is the one piece that implies the most that she has to be related to my family.
BUT!
Her father is where things get complicated. I have his name, wife, and birthplace. What I don't have is where he died, or any of this siblings, or the names of his own parents. This is the problem. Without knowing who his father or siblings are, I can't figure out who he is related to!
I so very badly want to figure out if Nannie Ferguson is related to my line of Fergusons!
I've studied the dates I have tracked down very closely. All details indicate that Ammon Mercer met Nannie on his mission, married her, and took her back to Utah shortly thereafter. Sadly, it appears that she passed away a year later.
My guess is that she was a second cousin to my great-great-grandfather. But since I can't find the great-grandfather to either of them, this is nothing more than a guess. I think it is a good guess by the fact that a man named William Griffin Ferguson was one of the first people baptized in Roanoke, and he was the second cousin to my great-great-grandfather, William Harrison Ferguson. (See, I told you they really liked the name William!) And William Griffin was born in the same small town that Nannie was.
But alas, I'm stuck. I can't seem to figure out how to relate Nannie's father to William Harrison's father. All of the similarities in the world just don't matter if you can't prove them! (Did I mention Nannie's father is John W. Ferguson? See! Another W!!)
What kills me more than anything else that is in a family the size of mine (William Harrison has well over 2,000 descendants) that no one seems to have noticed Nannie before.
AAGH!
Darn you family history bug!!

PS- ignore the marriage date for Nannie and Ammon in the image above. That is the date their marriage was performed in a Mormon temple long after her death.

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