Require him to come to the CONUS and if the evidence he provides does not pan out - revoke his immunity agreement and jail him.
It’s not an immunity agreement he seeks.
And, if what he has is what he says, it’s worth a full pardon.
What he has can be verified in advance.
No matter what information he has the swamp will say it’s nothing. His only hope is to take it to the media but say he won’t testify under oath until he’s publicly pardoned.