Those pardoned can invoke 5th amendment and refuse on that basis but if subsequently given transactional immunity they are compelled to testify. If they refuse to testify to Congress, it will expose them to contempt. If they lie during testimony, they can be charged with perjury.
Even if these criminals have immunity, we need to get to the truth and hopefully others will be exposed.
It’s the price you pay to get the truth from these government crooks.
Actually, in 1915 the Supreme Court decided that if a person accepts a pardon, they lose their 5th Amendment protection regarding the crimes for which they were given the pardon.