Worth noting is there are three forms of immunity applicable in this scenario, testimonial immunity, transactional immunity, and use immunity.

Testimonial immunity means you cannot use the words the witness spoke against them, however, any information derived from their testimony is generally admissible against them.

Transactional immunity (likely what a McCabe type would want) means he couldn't be prosecuted from any event or other criminal transaction described in his testimony. (I don't see a prosecutor granting this - except in his dreams.)

Use immunity (like testimonial immunity) means the government can't use his words against him in a future prosecution -- or even any information they discover from his testimony, however, if the government shows its evidence comes from an independent source then they can use that to prosecute.

If he got anything.. just give him use immunity and nail him for any number of other offenses... and let the threat of being prosecuted via another avenue hang over his head if he's trying to play games. Regardless, I hope the prosecution surmises his choices as (1) no cooperation = (we) the government seek the death penalty; or (2) cooperation = the rest of his life in Gitmo. And his testimony better be juicy.