I recently finished Warbreaker, and I thought it was awesome! The only problem I saw was that God King does seem a little too similar to Lord Ruler. God=Lord, King=Ruler. But other than that, the magic system was ingenious (as always) and the plot was amazing.
We don't know if the religion's interpretation of the Returned is correct. By making them rulers, they can influence things, though. The Returned may be imprisoned, but I must admit that it is a very nice imprisonment with a decent amount of power. Note, however, that their quest may not necessarily be to heal somebody. That may just be an act of mercy they perform after finishing the quest. So asking if Denth and Vasher have to heal somebody is not as relevant ask asking what they were supposed to do. And for Vasher, I think the term Warbreaker may be sufficient. I dunno about the others, though. They seem to have failed...
I got the impression at the end of the book when Lightsong healed the God King that he sort of knew it was time to do what he had come to do. My theory about the Returned is that they are very much like awakened sentinent objects (like Nightblood): they are given one command and that command sort of defines them. In the case of Nightblood, the command is "destroy evil" and it was given to him by Vasher and Shashara. In the case of the Returned, I think that the person gives
himself a command, causing his body to be awakened as an entity. So the 'quests' of the gods/returned are actually just a command they gave themselves. I agree that it definitely doesn't have to be healing someone.
In the case of Denth and some others, it would be possible according to my theory to have an evil quest, if the person gave themselves an evil command. However, since Denth used to be good, I think this is unlikely. Instead, I suggest he has an obscure command, like Nightblood's "destroy evil". Certainly Vasher could be considered 'evil' in Denth's mind.
On the subject of Vasher's and Denth's abilities to do things that most Returned can't, I would point out that they have been living for a far longer amount of time than the others. Also, the priests do hold quite a bit of information back from the gods living in Hallandren.
I totally agree that Clod is Arsteel.
Now, I would like to propose that there are actually 8 BioChromatic entities: four spontaneous, and four non-spontaneous - spontaneous meaning awakened and commanded by oneself and non-spontaneous meaning awakened and commanded by others. So the 4 non-spontaneous would be Nightblood-type things, awakened objects, lifeless, and a sort of sentinent creature created from a dead human body the same way Nightblood was created (this last one would be very dangerous - imagine Nightblood as a person). The four spontaneous entities would be Returned, returned lifeless, returned objects, and returned sentinent objects (if Nightblood was destroyed, he could choose to return as the last of the four). A returned lifeless or returned object seems improbable, since they would have to be awakened by themselves, but if you commanded an object or lifeless specifically to return if it was killed, maybe that would work.
One question: why didn't Vasher just use his own army to destroy the Lifeless army? He could have stopped them on his own, without the God King's help.