...because there's always one or the other source hunter who keeps saying that there are still some business undone in Rivellon BUT I can't finish these quests because the npc are dead ! Such a stupid broken game ! Plus I have no idea as to know to finish the quest of the general's story. I've read all the ancient letters and journals spread everywhere in the end of times.

Here are the quests still unfinished :

1. A Source-Hunter's journey
2. The General's story
3. The missing threads (I guess it's related to quests 2 and 4)
4. The General's story
5. Prisoners of the cristals (I killed Haizea instead of giving him what he wanted me to fetch but the quest is still says to get the spell)
6. The sad truth (I can't help Nadia because Brandon never gave me his quest and never got the isolating cage from Lawrence)

I translated the titles of the quests from french so the english titles may not be 100% accurate.

Thanks for your clear and helpful answer. I just beat the game after all this struggle to understand what was wrong in the process.