Also, why did the heavily pro-Union, Republican tradition in eastern Tennessee never spill over into western North Carolina?
The Tennessee Valley is the only thing I can think of that caused it. You have the Appalachians (pretty rough terrain even today) that does a pretty good job of separating North Carolina from Tennessee and the Cumberland Plateau that rises up and separates the rest of Tennessee from the Valley. I've always been fascinating by the little pockets like these in the South; I never noticed the "nub" as referenced in the OP, but it appears to be even more ardently Republican than my area in the TV.
My dad's side of the family is from Hardin County and I'm very proud of this area's Republican tradition 
You will note that these counties (Hardin, Wayne, Henderson, as well as Carroll and Weakley) voted against treason just like East TN:
Awesome! My county was "divided". Would've been cooler if they'd got in line with the surrounding counties that were against secession. It's interesting how there's a trail "Against succession" that follows closely with the Tennessee River. That might be one of the original components to why the area in West TN is the way it is.
