Heinberg wasn't even born yet when Wonder Woman's Diana Prince was created, in 1942. It's always been an integral part of the character
No, it hasn't. It was ditched in the post-Crisis reboot in 1987, and she went without from then until 2006, quite fine, because it's completely unnecessary to the character and, in particular, totally at odds with the characterization that was developed from George Perez onward. It was brought back because of pointless nostalgia.
Yep, and the reason? Wonder Woman, you are not human enough, you do not understand people. (Total bullshit but still.) So, you must take on a human secret identity to get in touch with your humanity, to understand average humans. And what was the job she was given "to get in touch with average normal humans"? ...secret agent... in a white formfitting jumpsuit...
Yeah, makes total sense there.