To the best of my knowledge (and I have been out on vacation the past 5 weeks) there is no way of doing this (ie skipping over to Ishad). You need the signet of the Mavin (at the end of the crypt) before the oracles will let you pass through the caves. By the time you get to the end game, the wait for thief guild level will mean very little to overall character development. The general recomendation is to avoid the rogue class as a starter, and simply rotate a warrior through to Barbarian to get your lock picking base.
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