Soobin's death was the result of an assassination plot from an enemy nation. Soobin's nation and the enemy nation is the same two that were at war during the duration of the fic and ultimately destroyed each other.

As for Kai and Soobin, I would say yes. Soobin would have landed in Elysium due to his heritage and his divine blessing. I think they would meet on accident - maybe one day Kai takes a little stroll around the fields the same time Soobin is getting away from the frenzied party he was dragged in to (as he was in life). They would meet again in the rolling fields. Soobin would be in shock - wondering how much time had passed and why Kai still looked so young. Kai would look at his old friend with guilt - he had outgrown the prince, Soobin still retaining his young glow of early adulthood. Eventually, they would embrace with tears of happiness and sorrow towards each other. Soobin doesn't blame Kai for his death - he never did.