The U.S. Constitution says that treason will be tried in the location where it was committed. To be true to the Constitution, they’ve got to try him in Richmond. That’s a terrible spot for the federal government to try the president of the Confederacy. Not only because the jury pool is going to be tainted against the federal government but if you find people who are loyal to the Union, to try the Confederate president, there’s going to be the intimidation factor, especially for African Americans. That’s very real in the South after the Civil War.
Wirtz was not in the union military. They invented a rationalization for doing what they wanted to do whether it was really constitutional or not. Imagine that.
Richmond was teeming with federal troops in 1868. They could certainly have tried Davis in Richmond if they had wanted to. The notion that they didn't try Davis because they were scared of confederates is just laughable.
They could have packed a jury any way they wanted. Reconstruction began in 1867 and disenfranchised most of the white population so they weren't going to be eligible to serve on a jury anyway. So much for that excuse.