Some of the Stern recordings were getting play during the primaries, but they weren't slowing Trump down since his opposition was split among so many different candidates.
The bus tape was archival footage. If you're suggesting that every hour of archival audio and video of Mr. Trump should have been sought after as soon as he became the nominee, then, again, you're underestimating just how volatile and fickle the news cycle is.
The plausibility of "the official story" (to reiterate: that the footage became an object of interest for NBC after former cast and crew from The Apprentice came forward to disclose his ill manner on the set of that show) seems pretty plain to me. Occam's razor defeats your conspiratorial thinking without breaking a sweat.