Pleading the Fifth AFTER being granted immunity should invalidate your immunity agreement.
You have nothing to worry about incriminating yourself.
In theory, once granted immunity for anything you say, you should spill your guts. Then they cannot prosecute you for anything.
Do I have this right?
Immunity from prosecution is one thing...there is no known immunity from Arkancide.
Also, immunity can be revoked if the person granted immunity refuses to answer questions or answers dishonestly.
If his immunity agreement is with the FBI (DOJ), it does not confer immunity against self-incrimination in testimony to Congress. That would require a separate agreement.
Consequently, he is within his rights to take the fifth in a Congressional proceeding.
He had some kind of limited immunity from the FBI. I don't believe we've ever actually been told full nature of his immunity, or what was gained by the FBI from it. (if anything). He doesn't have immunity granted by congress. If the congresscritters had actually been interested in the truth of what actually happened, they'd have granted him immunity as well. After all, he's just a low-level criminal who, in the big scheme of things isn't actually important. However, neither congress nor the FBI is interested in the American people actually finding out the truth about what went on, so he was allowed to take the 5th.
Bottom line: if the republicans actually wanted the truth to be known, they would have it.