holy crap, i never put it together that nadan might think suu was the one trying to kill the prince -- or even worse, that suu might have framed him. i don't know if he's that far in his distrust of suu but it wouldn't be horribly unjustified. nadan knows first-hand what the consequences of scheming in court are, and it's not just you who get exterminated, it's your whole bloodline and everyone you know, so he wants nothing to do with it. the only thing i disagree with you on is that nadan had a privileged life before becoming a traitor by association, because suu says nadan was never even given a name and lived in the slaves quarters. that implies he was considered a slave since birth, not as punishment for treason, and nadan is shown getting hit by other slaves in flashbacks so he may not have received special treatment.
nadan has a huge reason to hate suu that nadan supposedly still doesn't know about: suu made it so nadan would not be offered a place in the guard after his skill in the arena was noticed, because suu didn't want them to be separated. a pretty damn selfish play on suu's part. i think the nadan/suu and suu/sahara parallel is pretty solid, with baby suu and sahara both being the ignorant outsiders clinging to an older savior. think of when suu is chained to his bed after asking to see nadan and lashes out at sahara, and sahara replies, "but i like you like this. this way you won't go anywhere."
suu's possessiveness of nadan is not too different from the way sahara treats suu. suu and sahara both want to protect and repay the one who was kind to them when they were vulnerable but they're willing to sabotage their loved one if it seems like they are trying to break free from the dependent relationship.
i think this exchange between jin and suu in chapter 49 says it all about suu's perspective:
jin: I did notice that something about him makes you act irrationally.
suu: would you believe me if i told you that...certain memories can take root inside a person's heart, and control his entire life? Little moments that seem trivial in the eyes of others but which keep him going for the rest of his life. Binding him to the past, never letting him go...
Ah, you could very well be right about Nadan's upbringing! Sometimes I get the timeline confused. Was Nadan a slave even before his father commited treason?
Another thing I want to put on this Nadan thread:
First, in chapter 60, when Suu goes to retrieve that green sacred book from the library and runs into a young master from the Heo family (whom Suu served in Jugang), the young master said that his father's friend would have paid for Nadan's release from slavery, but Suu begged the young master's father not to do this. And never told Nadan that Nadan could have been free. The REAL twist is that in chapter 82, when Suryeon's two guards are pondering WHY Nadan became a slave fighter in the arena to begin with, they theorize that MAYBE it was to win Suu's freedom from slavery. (Now here is My theory: that Suu purposely may have stopped Nadan from being freed by the young master's father's friend so that Nadan could first win Suu's OWN freedom by fighting in the arena. It's a stretch, but I could see it)
Also, it is highly possible that Suu had to murder to keep Nadan safe, which is mentioned later in chapter 64. Since Nadan's father committed high treason, which is punishable by extermination of 9 generations of family (so this includes Nadan), the prince implies: 'Is this somehow related to a few murders you commited in Guiju? (We find out later that Suu has killed for Nadan, chapter 86)
Ok, so now more up to speed in chapter 78, when Suu goes to visit Nadan, Nadan seems overly joyed to see him, which is a stark contrast to the cold shoulder he had given the previous time that Suu had gone to visit him in his imprisonment. My thoughts were the same as Suu's: Suryeon is somehow making Nadan (probably via torture) seem welcoming. It was like he had been asked to embrace Suu with open arms even though he has strong discontent for Suu at this time.
Remember, Suu pulls away and asks Nadan: 'They are threatening you, aren't they?'
I, of course, was wrong, as was Suu. Suryeon had promised Nadan escape from the city if he acted fondly towards Suu upon Suu's visit. When asked if he would like Suu to come with him, he firmly says that he has no intention of spending the rest of his life with Suu. I think Nadan believes that his worry over Suu needs to end, as does Suu's worry for Nadan, to put it in simple terms.
Ok, so I am so sorry if I rambled on, I just really wanted to paint a timeline, mainly for myself to figure out the sequence of events. Always open to more theories!

Ok, so I reread the whole thing and I want to take a minute to talk about Nadan, as he was a character I struggled to understand and I think I got him figured out.
I am completely open to disagreements here! To be honest, I am still on the fence about him.
Anyways, what I think it basically comes down to, is that Nadan had a privileged upbringing (even if his mother was a slave, his father had political power), until his father was charged with high treason, but his comfy childhood made him blind to the harsh realities of the world and made him ‘kind’. He is someone who does not ‘scheme’ and is completely normal, all of which are traits that Suu strongly envies.
He once loved Suu when they first met as Suu arrived in Rahan, but Suu is a strong-willed person, always trying to get ahead, etc, and the opposites of their temperament just became too much for Nadan. This became especially true when Suu encouraged Nadan to become a warrior. The harshness of the arena changed Nadan, and made him resent Suu.
Still, Nadan had strong feelings for Suu and it is in his nature to care for Suu and protect him. Even in the first few chapters, he constantly asks if Suu is ‘staying out of trouble’ and would dread the idea that Suu was working for the noble consort.
Still, when Suu was taken away to the shrine to care for Saharah, Nadan barely noticed his absence, which you can tell greatly hurt Suu, but this showed that any romantic passion that Nadan had for Suu has been gone.
Now, to Nadan’s imprisonment: when Nadan and Suu manage to meet up (after Suu’s placement in the shrine), Suu is afraid that the consort is looking for Suu and angry with him for not poisoning the horses (from the first few chapters, this was his task) and tells Nadan ‘not to mention him if someone asks’. Later, when Nadan is framed for attempting to assassinate the prince in the forest during the hunt, he remembers what Suu had said, to ‘not mention him’. Nadan automatically believes that this ploy, ‘to kill the prince and frame Nadan’ is Suu’s doing. This is why Nadan is sooo cold to Suu when Suu visits him in his imprisonment. He knows that Suu gets tangled up in dangerous situations, and he believes that this is just another situation, and he is paying the consequences for Suu’s actions (even though we know that is obviously not the truth; it was the consort who planned to assassinate the prince during the hunt).
Without Nadan, Suu might have died in Jugang when he first came to Rahan, and these memories are what makes Suu so fond of Nadan. That, and Suu’s envy towards Nadan’s naiveness. Overall, Nadan gives Suu something to fight for, an innocence that he wants to keep safe, even if they do not fully have romantic interest in each other (as I recall, when asked if Suu would take Nadan’s place in prison, his answer was obviously ‘no’, as he does not love Nadan more than himself)
I think that's all I have to say about him for now!