Interesting about the OT on the Celebi, since you work at GameStop you should have gotten the new WIN2011 (02211) distribution cart as opposed to the GAMESTP (02271) one.
It's got the Fateful Encounter flag, so you should be able to do the event so long as nobody else has done it, namely your brother! There were two different Celebis that were hacked that had the Fateful Encounter Flag, so the event could be done before any official distribution. Be sure you walk all the way to the shrine, and talk to it if need be (I haven't done the event myself).
Changing the date on your DS wouldn't cause any corruption of any kind. It's a common trick RNGers use called "Seed Hitting" (off topic)
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