I agree to keep it simple, but there's the powers. No amount of super-sekrit training by lady warriors is gonna make an orphan girl able to lift a tank or fly or move fast enough to knock bullets out of the air, IMO. Either something like, 'she was near death after Diana saved her from the [deleted] that killed her parents, and the Amazons were only able to save her by using the Purple Ray mcguffin to transfer some of Diana's immortal life-energy to her and somehow she got powers like Diana's out of it! And they can never do it again, for... reasons.' (Because we don't want Diana doing this to hundreds of people!)
Or some multiversal version of Diana from a lost reality or something. Maybe a creation of Wonder Tot, the fifth-dimensional imp that wanted to adventure with 'teen Diana' and adult Diana at the same time! Might as well go wonky with it.
Quantum Donna. "I have *all* the origins! I was rocketed to Earth after my home-dimension fell to the Anti-Monitor, only to discover that my people have great strength, speed and flight on this world. My parents died in an arsonist-set apartment fire in Gotham, and I ended up seeing a poster of Wonder Woman and abandoning my family legacy to wander the Earth and learn her secrets so I could bring justice to the world. I was hit by lightning while analyzing some Amazonium fragments I'd found at a crime-scene. A dying Amazon from another dimension named Abina Sur passed down her lasso and powers to me."
Just keep it short, a few sentences or a panel at most, and *change it every time*, with Donna giving no clue whether it's a long-running gag, or she really believes all these wildly different origin stories...