Bill 9000, I know your Earth-2 Legion of Super-Heroes doesn't have a Superboy or Supergirl, but here's an evil suggestion -- we know that Kara Zor-L arrived late on Earth-2. What if during those "missing years" the future Power Girl was spending time in the Earth-2 Legion as
Andromeda?
This ties into the first arc of
JSA: Classified, where the Psycho-Pirate overloads poor Peege with wildly different origin stories. One possibility was that Laurel Gand was sent back in time by the Legion in an attempt to foil some Mordru plot. (This also jibes with the since-abandoned hints that Karen Starr was "living her life backwards".)