Caius Cosades won't talk to me
Solution 1:
Well, as your comments reveal, somehow, Divayth Fyr is dead, which means the game is working as intended, and to complete the Main Quest with this character, you'll have to take the Backpath, as documented on various places, like the UESP Wiki.
Or just reset. Since it seems like you're only at the Deliver Package part, so resetting isn't hard? Unless you spent a LOT of time wandering around beforehand?