Starting with the easy questions, there are no more Demon Hearts available in SoA. But if you have ToB installed, there are a couple in Watcher's Keep.
It is in fact quite possible to beat Chapter 5 without Resting. I have frequently done all of Chapters 4 and 5 at a run, not Resting until I get back to Athkatla.
The fact that your game crashes ONLY when Resting in the Underdark leads me to believe that either the correct Rest movie for the UD is corrupt, or the area file for the UD itself is corrupt. Try Resting in the Svirfneblin Inn and see what happens.
And the fact that removing the Cleric from your party causes the game to progress normally points to a corrupted *.CRE file, or even a corrupted Savegame. But these would most likely cause errors in non-Underdark situations as well.
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Volothamp's Comeuppance
Everything you ever needed to know about the entire Baldur's Gate series......except spoilers.
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