Sure they were. Though if memory serves some of the German officials were sentenced to death for their use of slave labor. Rebels leaders should be glad that there were no post rebellion Richmond Trials I guess.
It would have been awfully embarrassing to admit inconvenient facts like the Grants owning slaves at the time not to mention the many other unionists who owned slaves or work on the capital dome during the war being done by slaves.
President Jefferson Davis demanded a treason trial after he was incarcerated. It was decide that to put him on trial would put the issue of secession on trial. The USA thought they would lose the case and that would be to embarrassing. Davis was eventually let free.
It sure would...except that like so many other claims of yours there isn't a shred of truth to it. Grant owned a single slave which he freed in 1859. After 1864 Grant and his wife didn't live anywhere where slavery was legal. The claim that he owned slaves up until the ratification of the 13th Amendment - and some lost causers have even claimed he owned them after the ratification - is complete fantasy.