Tennessee might well have reversed itself in the post Sumter 2nd election. But the process was so irregular, I believe that we cannot know for sure. With an alien army, the CSA, already invited in and widespread voter intimidation in the more slavery-oriented regions, there is cause to doubt. But no matter how the people would have voted in a free election, it was not for Governor Isham Harris and his political henchmen to disregard the February vote, the last vote on record, and take it upon themselves to invite the Confederacy to overrun the state.
I think the general lack of super majority support for Secession came back to haunt the CSA late in the war. The hardcore true believers of the Confederacy had volunteered early and tended to be sent to the immediately critical Virginia theater. The reluctant warriors tended to be more in the West and that's where the Confederacy fell apart while the do or die boys were still fighting for a stalemate in the long battle for Richmond. And the presence of a large unenthusiastic segment created a critical base for erosion of the general support for the war when the going got tough.
I think the disregard of the Constitution that you rightly decried would be a danger no matter what path Lincoln took. The heirs of the Confederacy, the southern Democrats of the New Deal era, were generally as much for intrusive government as anybody when they thought it could further their states.
It's a pleasure debating differing POVs with somebody who appreciates the history.
It is amazing to be able to go back in history and see when people actually believed in the Constitution and were willing to sacrifice everything for it regardless of what side of the issue they were on. Most people today do not care nor do they realize what they are giving up by allowing the government to incrementally remove their freedoms.