... because if the DA decides *NOT* to tamper with evidence in 'this specific case', then he goes to jail anyway for concocting this elaborate hoax to try to accuse the DA of doing something that they didn't do (in this case).
Except the DA couldn't have won that case without planting some form of evidence that wouldn't be considered consequential. CJ knew that if none of the evidence against him would actually seal his conviction, the DA would have to plant something that would. And considering he wanted to be governor, the DA couldn't afford to risk losing a trial. CJ knew the DA would do whatever it takes to put him behind bars, including planting evidence that couldn't be argued against.
Unless the DA didn't care about being governor, it was a sure thing.
I also don't see why he had to go through with this elaborate hoax. With a dirty DA, why not just assume that somebody else will get away will be framed by Michael Douglass? That probably has a better chance of succeeding.