Like i said, this is all in concepts, i have no doubt that many things will change.
Also, preoject development/importance is extremely subjective. i could say the next fifa MADDEN game will be the greatest game in development for any engine, its oppinion. while king's project may be fairly awesome, it is not indisputably the best project simply for the fact that it is oppinion not fact. (same goes for my, and all other projects)
NOW. you seem to overlook that it was NOT sylvanas playing the campagin through, and there are many things she does not know. Also, vengance and power incentives are great, and espcicially if this pact decrees somthing for her when arthas is killed, they have no reason to destroy her when it is done, because she is a valuable tool to them.
She has many a reason to make a bargain with tbl. and she cannot kill arthas on her own, she needs the demons to help her. kel'jaiden and her both want power. (and the combination of the two may blind her to the possibility of traitorisims.)
But again, its in concept, and i have alot of other reasons to go down this path aswell. there will be lots of interesting things going on.
Also, tbl has no reason to destroy azeroth, it is an illogical waste of energy.
and if titan interfeance is all that they want to purge, there is little reason to go and focus on azeoth in the first place. there are no doubt much greater fish to fry, so to speak. they want the well and its powers, and they want to enslave the lich king and arthas once again (again, a possible betrayal for the forsaken, she wants arthas dead, they want him under their knuckle, he is awfully powerful, ofc, not cosmically, but...)
i can see a few reasons. for doing everything, and an argument can be made for practically everyone. humans, and sentient beings in general, have not been know for making the best decisions ever, so i see no reason why none of the charachters in this mod will make only the perfect decisions.
Thanks for your oppinions everyone, and especially king.
And also, we are not focusing on this part of the story at the moment, so i would suggest you let it rest for now.