There is no way that Shallan gets her happy enging in book 3 in a 5 books series. That will meen no more progres for her from now on.
From what we have seen in this series so far Radiants need to solve some kind of dilemma to advace in their oaths:
-Shallan- to recognize a truth about herself: 2 I'm a killer, I killed my father, 3 I killed my mother and caused the destruction of my family; 4......don't know, someting even more important then 2 and 3 maybe something like " i am a lie -everything I showed since I was a kid were just faces who helped me cope the world" Shallan the nice, smart, jollyful daughter was enough until she needed to become something else: a spy so she created Veil, then a radiant so she created Radiant, but none of them are real just faces to hide the real self.
Shallan can not achieve 4 as long as there is something that stabilizes her as it is now, with Shallam as the main personality and the other 2 as secondary. Adolin is the easy choice that gives her stability, comfort, so she does not have to analyze even more serious truths about herself, he makes her happy with the face that she is now. Even if the 4 truth is difrent from what i belive,Shalan will still have to be placed in a very difficult situation to admit the next truth- I do not see how it is possible as long as she is happily married with Adolin. So he will have to be removed. Adolin's death, perhaps sacrificing to protect her, will make her lose control again and be forced to think who she is without him. And no divorce or some other form of separation it will not be enough
-Kaladin- a moral dilema:2 to choose to protect, 3 to choose to protect those he hates, 4...... I do not know, but it's probably something conected to " not leting his failures to keep protecting those that are alive" or maybe " to let others sacrifice themselves for the things they believe in and not protect them"- something harder then 2 or 3.
And BS has begun to put him in such situations in OB (humans/parshendi, the 2 grups Kaladin traind facing eachother....) but they were not strong enough, it will be necessary for someone very important to him to sacrifice and lose hi life, like his best friend, his only real friend.
That's why I think Adolin will die in the future book, maybe after he succeeds in reviving the sword and saving friends with her at a critical time. That does not mean that Adolin must be removed to make room for a relationship between Shallan and Kalladin, but that he neded to become important for bouth of them so his death will be a critical moment in there evolution to full Radiants- part of there "journy".( and I can see BS doing this to Adolin, hi is just a secondary caracter, placed in the story to suport others, the most important secondary caracter, but I think this is also intended for his death to have a stronger impact). It does not necessarily have to be a romantic relationship between Shallan and Kaladin after his death.
That's just my opinion about how the story will evolve in the future, I might be mistaken, and Brandon will let the marriage work, Sallan to heal better with Adolin by her side and tell the rest of her truths happy and with smile on her lips, but the story has not been so until now. Unfortunately I think Brandon will get our heroes to go through a lot more pain until the end of the series.
And Dalinar will be afected by his death to. I don't think there is another caracter whose death will have so much personal impact on the rest (Dalinar, Shalln or Kaladin deaths will create huge problems but not so big emotioal responses from others as Adolin's will)