My guess is CBS has either an in-house document verifier or oen they use regularly. He probably serves as a rubber stamp, doesn't actually look very closely. The attitude at CBS would be, "We need you to issue a favorable opinion of the documents' authenticity so we can run the story." If the expert is a team player, he'd feel pressured to verify the documents.
No matter what else it is, journalism is a business first, and they felt they could make advertising money if they received verification of the documents.
Your scenario could be right. But whether CBS used in-house, out-sourced or no "experts," it's clear that they made no serious effort to authenticate, much less to "thoroughly investigate." They are liars and Dan Rather is at the top of that list.