The date of the letter from the warden to Mapes was November 20, 2001. McVeigh was executed on June 11, 2001. If she was working on a McVeigh angle, she was a little late. Of course, there's no date mentioned in the letter as to when she was doing her little favor for Peter Langan. She may have been in contact with Langan for an inside angle McVeigh prior to his execution--but he had been transferred to Terre Haute a number of months before his execution.
McVeigh alive wouldn't do, if Mary got her story from Langan.
It would be after he was dead. MM would get Langan to give her a story by doing these favors. That story would be whatever Langan saw made Mary happy. Which would likely put blame on all MM's enemies (which included both father and son BUSH)
Another, "We don't just report the news, we make it up" story to CBS and MM.
No one could ask McVeigh for confirmation, now could they?