Trump can't touch state convictions and state prisons work perfectly well. Last time I checked, almost all executions are carried out by states rather than the federal government.
As long as those convictions are state convictions and contempt charges are in state courts, Trump can do nothing. Pardoning people who can testify against you (IE. in the same conspiracy) is a huge tactical error: You just immunize them from federal prosecution which means they can't take the fifth amendment and refuse to testify. They will be required to testify or go the jail for contempt. If those are state courts, Trump can do nothing about those contempt charges.