The Fisher book is excellent.I also recommend Daniel Ellis' (the Red Fox you referred to) biography, "Thrilling Adventures of Daniel Ellis."Both books, and others (including Temple's history of the Civil War in East Tennessee), are available through Continuity Press.
You ask a difficult question about what made E. Tenn. unique among the unionist regions.I don't know enough about the other areas to compare them.In my view, the existence of the major railroad lines through E. Tenn. was a major factor.First, it was primarily merchants who used the lines to trade with the south that were confederate.To some extent, they were important people as opposed to the "common" men.Secondly, the lines made the region of extreme strategic value.Third, if the Union general had kept his commitment and invaded E. Tenn. immediately after the bridge-burnings, the plans were for a independent state of E. Tenn., just like the separation of West Virginia.
After the burnings (and no invasion), the Confederates cracked down, causing reaction by the unionists.Pro-Union Families who would have stayed "neutral" in their actions were compelled to step forward, especially as the conscription efforts were increased.The bridge burnings brought the undelying divisionsout into the light and from there the folks became more and more polarized.
Another factor may be the disaster which befell the 2d Tenn. Mounted Infantry (my gg grandfather's regiment).Around half of the regiment was captured in the Battle of Rogersville.Many of those men died in prison.Non-combatants were also imprisoned.Hard feelings grew harder.
Perhaps another circumstances was the proximity to Virginia.I believe many E. Tennesseans migrated there from southwest Virginia, not just for "greener pastures" but to escape Virginians.As a native of the region, I think there still is a big difference in the people.
This just a start.A question I will ask is this:what was the typical number of union units from the other southern regions?I can't say right now how many E. Tenn. units there were.I have ancestors in at least three different units.