If there is any evidence that Sessions' calls or e-mails are among those that were intercepted under the FISA warrant that is the subject of a criminal case, his name is going to appear on the witness list even if he never gets called to testify.
As soon as his name appears on the witness list, he cannot stay on the case unless he intends to argue that he is not a material witness in the case. There isn't a judge in the land who would agree with that argument. More importantly, he would come off looking very foolish and unprofessional simply by going to any great lengths to stay on the case.
As I said before, I disagree that the substance of the calls intercepted and the various parties thereto would be considered relevant evidence for a prosecution of Obama and his team for the crimes I indicated for the reasons I gave.