If you remember the events of Brood War, you will clearly note that Kerrigan's forces decimate the UED after the entrench themselves at Earth in order to rebuild. Therfore, Kerrigan knows where it is, the UED knows where it is, and unless they've stored that knowledge in "Magic-Town," I'd imagine that someone else could discover said tidbit of information without too much of a struggle. And honestly, does it make a whole crap-ton of difference where the setting takes place? Honestly, that was a quickly thought up little blurb of a story and is definitely going to be subject to change, but instead of focusing on earth Vs. Korhal or Char Vs. AnotherPlace, can I ask if you guys are okay with me doing this?
Not that I want any change in your otherwise brilliant idea, but dude. You got it all wrong. The UED sends an expeditionary fleet(and manage to take the terran colonies, the Zerg AND the protoss). However, Kerrigan manage to retake control of the Zerg broods and decimate a large part of the UED fleet. The rest retreats to Earth, however they are annihilated by the Zerg long before they manage to return to the UED homelands and Earth. There's no way in hell the Zerg could successfully invade the UED. Remember, when an expeditionary fleet can take the entire Terran quadrant, then imagine how badass REAL fleets of the UED are.
(I might be overreacting here, but I've never gotten the opportunity to discuss Star Craft before )
However, I DO love your setting, and I truly wish for it to be done.