you can finsish the game with anyone in your party becos many ppl play it solo and some as mages so that answers the first question. ask around for soloers who have done it.
and secondly I know you can go back in the underdark but I have never tried reentering the city but I assume you can so give it a try. and dont forget to have fun with the mindflayers
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"I was born of darkness. My fathers eyes closed before mine opened. I am not of this world or the other, and I have the right to be what I am..."