“I remember reading that the government likely dropped the treason case against Davis because secession was - at the time - constitutional, and letting that play out in court would have created another national emergency.”
It’s interesting that Charles Francis Adams Jr thought so as well- he was a Union officer who fought at Antietam and Gettysburg and was the grandson and g-grandson of the two Presidents from Massachusetts- so hardly someone who could be accused of Confederate bias.
Some years after the war he decided to investigate the legality of secession and gave a speech about it to Phi Beta Kappa: “Shall Cromwell Have a Statue?”- hard to find a copy of this speech anywhere but worth reading if you can.