I need to check some in-game literature, but I am pretty sure that Lucian was never a Sourcerer. He was a Marked one in Divine Divinity and then because he was capable enough the gods dumped their power on him. Otherwise I have no explanation for his well-being in this ending. Although if you give up your Source Malady manages to restore your mind - maybe Dallis did the same with Lucian?
Btw about Dallis - the Eternals were like this (resistant to death I mean) even before the Gods have harvested Source, so I'm pretty sure that they don't need it to continue their existence.
The only time I managed to save Gareth is when I spared Alexandar and convinced him to help me become the next Divine. If Alex dies on Nameless Isle because of you, Gareth is pretty much screwed.
