The good news is no they cant refuse to answer questions by congress. There is no 5th amendment protection when you are pardoned - since you cant be prosecuted.
Therefore, they must answer truthfully about anything they are asked. If they lie, a new crime has been committed for which they are not pardoned - then you can convict them for lying. If they tell the truth - we get to see under the rocks of all the bad things they were doing.
And, a RICO suit would be very appropriate.
Is it true that you can’t be prosecuted? Or just that you can’t actually be forced to pay a sentence?