Maybe the party is only going to be locked in for Moonrise Towers as it has been for Spellhold and and you'll meet the others again in Baldur's Gates or something? If you had to sacrifice them that would really kinda suck (well except for Lae'zal I could sacrifice her anytime I guess)
That would make sense, yes. Kinda forgot you couldn't swap out party members at Spellhold (I never tried doing that anyway).
