I suggest you read "The Conspiracy to Implicate the Confederate Leaders in Lincoln's Assassination" by Seymour J. Frank, which appeared in The Mississippi Valley Historical Review, March, 1954, Vol. XL, No. 4. Frank's article was reprinted by The Beauvoir Press, Box 200, W. Beach Boulevard, Biloxi Mississippi 39531 in 1987. Beauvoir was, of course, the home of Jefferson Davis and is now a museum.
Stanton was the one who made the accusation that Davis and others engineered the assassination. From the article:
In support of Stanton's accusations, Holt [a longtime political ally of Stanton] promptly presented to the Chief Executive, in oral form, the stories of Richard Montgomery and Dr. James B. Merritt, two informers in the employ of the War Office. Although these statements had neither been made under oath nor reduced to writing, Johnson, apparently convinced by this hearsay evidence, immediately issued a proclamation which "startled the whole continent" ...
A number of other witnesses were procured by Holt and Stanton through the assistance of their chief witness, Sanford Conover. Some had testified in the earlier trial of the Booth conspirators. Here are some excerpts from Frank's article again:
Conover was then called to Washington and testified on May 8, 1866. ... Campbell [one of the witnesses against Davis procured by Conover] followed him on the stand and upset the proceedings by admitting his testimony was false. Conover, he declared, had prepared the statement which he had memorized and repeated to Holt. He readily admitted that he had perjured himself and stated that he had received $500 from Holt, $100 from Conover, and an additional 300 to cover traveling expenses. Conover promptly swore that Campbell was a liar. In company with a sergeant-at-arms of the [Congressional] Committee, Conover then left for New York to secure the attendance of other witnesses he had originally procured. On his arrival in New York, it was alleged, he eluded his guard and disappeared.
Dr. Merritt was called before the Committee, where, under cross-examination by Rogers, he admitted that his testimony before the military court was also false. He confessed that Conover had secured Montgomery as well as himself as witnesses for the prosecution, and that he had received $6,000 from the war office for his services and testimony. So startling and incriminating were Merritt's disclosures that the majority of the Committee [Radical Republicans] refused to allow the attending court reporters to transcribe the notes.
Letters apparently exist between some of the witnesses against Davis that show that the "arrangement" [with Holt] had been confessed to Davis' friends. One letter alleges that Davis' friends would pay large sums for testimony on Davis' behalf.
Holt apparently decided to make Conover the scapegoat for the failed attempt to get Davis. Conover produced some letters including some from Holt and had them published in the New York Herald and other newspapers. Holt replied by publishing two tracts entitled, Vindication of Judge Advocate General Holt .... Holt never sued the Herald for liable, but claimed the letters were false and denied being a party to Conover's fraud.
Conover was arrested and tried for perjury before the Judiciary Committee about his testimony there. He was convicted and sentenced to ten years in prison.
To promote his case for getting a suspended sentence or pardon, he approached James Ashley and Benjamin Butler offering to provide evidence connecting President Johnson to the assassination of Lincoln. He didn't get his suspended sentence, thought he had been betrayed, and sent to President Johnson evidence (letters, outlines of proposed testimony) of the plottings against Johnson. Johnson promptly published them in the newspapers on August 10, 1867. Johnson granted Conover a pardon in 1869.
However, Jefferson Davis was a traitor by any definition of the word.
He just was not a convicted traitor.