The dynamic between these 3 characters has been interesting throughout the story. A satisfactory ending for that relationship will determine how successful the conclusion of Akatuski no Yona is, in my opinion.
Yes, a good resolution to their relationship must happen for this manga to have a satisfying conclusion.
I think that Yona and Hak should forgive Su-won and some kind of reconciliation should happen.
Just to make it clear, when I say Yona and Hak should forgive, I don’t mean that they should forget and act as if nothing happened, which is something different altogether. It is more like they should move on and not stay bitter about the whole thing till the end of their lives.
Also, by no means do I expect that Yona and Hak should trust Su-won as they used to or for them to become friends as they used to be. By reconciliation I mean rather that they will be on speaking terms and nothing more, which I think is the most plausibly optimistic outcome for them. I don’t expect them to be friends again or to have the same kind of relationship like before the coup, because it is impossible.
Soo-Won's status is the most speculative of the 3. Will he live? If so will he be King? Or will he die EoS?
Su-won’s character started by defying expectations, one would rather expect an evil tyrant who eats babies for breakfast, so I think that killing him off, which is the most cliché resolution to his character, would be a terrible idea.
I think that Su-won should stay alive and remain the King. It surely would not be cliché, because most people, when they start reading Akatsuki no Yona, tend to think that Yona will take revenge and become the queen, so having Su-won stay as the King would defy readers initial expectations.
It only becomes apparent while reading that the story may be leading to something totally different.
Besides, Yona showed zero interest in ruling and has no knowledge in key fields like administration, politics, diplomacy, strategy and economics to be a good ruler. Su-won studied them, so he is simply much better qualified.
Hak and Yona, being the protagonists, will be there right to the end. Will Yona return to Hiryuu palace?
Yona and Hak finding out what they want to do with their lives. As for Yona, becoming the ruler will not be a good idea if she has no interest in administrative job, which ruling basically is, and she has shown no interest so far.
I think that being a ruler is not the pinnacle of happiness, so if she chooses something else, then that is fine.
Hak is trickier. However, recently Kusanagi has thrown at him some good bits of development, so maybe she has something planned for him. Returning to the Wind Tribe to take care of it and taking back the position of the Wind Tribe general? I don't know.