The information I have for my William Pool is that he was born in 1720. Regarding the two unknown siblings, with infant mortality rates in the 1700's I would be inclined to agree that they died as infants but like you entirely possible one of them moved farther west and started his own family and his information has been forgotten by the other parts of the family.
What year do your Edward born & death, and what are the names of his children? I have been corresponding with someone else I met from here named Audrey Pool, she is a researcher of the Pool family as well and I will tell her about your Edward once I hear back from you.
I did not see the NC county map on www.family101.com, can you tell me how to get to it? For NC County maps I have used http://usgwarchives.net/nc/ncmaps.htmlhttp://usgwarchives.net/nc/ncmaps.html which has a good indication of the boundaries thru the years. Specifically for Johnston County, I use http://www.rootsweb.ancestry.com/~ncjohnst/maps.htmhttp://www.rootsweb.ancestry.com/~ncjohnst/maps.htm which is more detailed and shows natural features like lakes and rivers and also shows towns (it takes a current map and overlays it with the boundaries at previous times in hsitory). But of course that does not help you for Rowan county, although that same website may have similar maps for all NC counties.