"How could Gerald Ford be deep throat, given that he didn't join the Nixon administration until well after the Watergate story broke?"
As a VP he was there all along. He didn't suddenly become VP.
Uh, actually, he did.
Agnew resigned in late 73 then Ford became VP.
Problem is, the Watergate story had already broken by that point. How could Ford be the source what happened before he suddenly became VP and before he was in the WH?