Is it possible that the appeal to the Supreme Court was a major concern?
On November 30, 1868, Davis' lawyers filed a motion to quash the indictment. A hearing was held before Chase and Underwood on Dec 3-4, and on Dec 5, they announced that they had split, with Chase wanting to set aside the indictment, and Underwood, who had overseen the grand jury responsible for the indictment, wanting the case to be tried. Chase stated for the record that he believed the 14th Amendment exempted Davis from further prosecution.
The certificate of division between Chase and Underwood was forwarded to the Supreme Court, and the indictment technically remained pending, but no more action was taken. The Davis case remained on the circuit court docket for February 15, 1869, but the government indicated at that time that it would not prosecute (nolle prosequi).
It would seem that Chase was clearly indicating he would favor overturning a guilty verdict and that could have made the government hesitant to proceed.
The government could not claim an automatic win in the Supreme Court. They must have learned that from Ex Parte Milligan.
Now, perhaps you will answer the question I asked of you a number of days ago. Is it your contention that the government knew it was acting illegally when military tribunals were set up in states like Indiana? Obviously the court ruled in Milligan that such tribunals were not needed in states like Indiana where the court system operated. But are you suggesting that the administration knew it was illegal when they initiated them?
Why should it be? The Chief Justice's desire to set aside the indictment in 1868 was due to his opinion that the passage of the 14th Amendment made any conviction of Davis an unconstitutional violation of his 5th Amendment rights.
Had the trial been held in 1866 then Supreme Court review was nothing to fear. Since the administration was insisting on a trial by federal court instead of military tribunal then the problems uncovered by the Milligan decision wouldn't be an issue.