This question stems from Can a person refuse a presidential pardon?
There are also other practical effects to accepting pardons, such as waiving of fifth amendment rights relating to the pardoned crimes, since it would be impossible to self incriminate anymore. (...)
And I went into the comments for clarification, but things got big enough to deserve its own Q&A.
If you get asked a question under oath which would force you to admit to a crime if answered truthfully, i.e you would incriminate yourself as to having commited a crime, then you can choose not to answer by "pleading the fifth" e.g. invoking the fifth amendment's protection against self incrimination. Most western nations have this right. If you get pardoned for a crime, it's essentially not a crime anymore, so you can't rely on fifth amendment protections to allow you to opt out of being asked about the details. – Magisch
Now the scenario, John Doe was accused of [crime], prosecuted and condemned to a death sentence by a jury of his peers, etc. Then was offered a presidential pardon and he accepted.
Interested parties can question him (where are the bodies, how did you enter the CIA database, what is the passcode to your smartphone we didn't return). Can he be forced to answer?