My take on it is thus:
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Sarevok is the NPC that asks to join you as soon as you arrive in the Pocket Plane...not Irenicus. Now, as far as Irenicus goes, I never encountered him...but I continued my game straight from the end of SoA. Does that make a difference? Or is it purely a level-based thing? Although that doesn't make much sense as everyone (unless they removed the XP cap...which is unsupported)starts at the same number of XPs. ToB automatically bumps you up if you are not already of sufficient level.
So...maybe if you import a character without playing through the final battle in SoA directly into ToB, this occurs. Any thoughts???
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Now where did I leave that doughnut?!