So. I did a full reread of books one and two last night (I’m a fast reader and I skipped a lot of slow scenes) after I finished book 3 and I have a theory on this!! I was reading specifically to see if I could find hints that Laurent knew. Here what I got:

Book 1

a. I definitely think he did know right away. I don’t have my kindle with me, but in the scene where Damen is presented to him for the very first time, Laurent’s face goes white enough and shocked enough that Damon notices, before Laurent quickly stamps that down and affects a bored kind of look-over. Laurent sends him away at first, too, and then suddenly says “wait,” and asks him to speak. He can’t turn down this chance to get to hear his brother’s killers voice; it’s probably a morbid curiosity. Maybe he is already plotting all the ways he can get revenge.
b. Then, that night, Laurent gets, again, noticeably drunk enough that Damen picks up on it. I made sure to confirm this, because in book 3, Damen says “You were drunk the night you got me,” and Laurent very clearly says “Yes, because of you.” Compared to all the other evidence of how Laurent comports himself, I have a hard time imagining any other slave in the entire world would inspire those reactions
c. Then, there is a very clear period of Laurent being VERY AWFUL. The arena with Govart, the beatings, etc. He knows he has to keep Damen alive for his own life (his uncle’s commands) but tries to make him miserable as possible. I think the bath is a test designed, purposefully, to show himself exactly how bad Damen is because Damen had surprised him with the remark about raping children, and Damen fails it spectacularly, so Laurent can have him “killed” and not feel bad about it
d. This is when it gets interesting. I think Laurent is torn up for most of book 1 because he hates Damen and what he did, and his uncle is forcing him to keep him around, but he keeps seeing these little signs of decency (the whole protecting slaves thing and Damen saving his life)
e. Also, I wish I had my book with me, but there are a couple scenes where Laurent is pretty clearly probing to get at information about Kastor that seem too pointed
f. Oh! And Laurent really does NEVER say the name “Damen” in either book one or book two. I think it hits too close to home.
g. I think he was planning to leave Damen and be rid of him, but then changes his mind at the very last minute because he doesn’t want to leave such a strong weapon in the hands of his uncle, and he really does think that Damen might help keep him alive

g. The sex is an extension of this. He’s exhilarated, because Damen helped him do something. “this is the first time I’ve won a move against my uncle”. Laurent has a way to think about him that isn’t the prince “My captain” he says at least three times.
h. Also, Damen is STARING at him, both when he enters the room while Laurent is being undressed, and later in the banquet hall. He’s had signs for weeks that Damen wants him “control yourself” Laurent said, back in the tavern, when they were in disguise. I think at this point, Laurent has known he wants Damen back for a while (at least since they were in the tent together the eve before the battle), but has been willfully resisting.
i. Then, Damen kisses him on the balcony. And he’s so slow and romantic about it. So respectful. At that point I think it’s already a losing battle for Laurent to deny his feelings.
j. But the interruption gives him space to think, to remember who Damianos is. That’s where the angry, desperate handjob comes from. That’s not only his frustration about the betrayal in his troops, but his vivid realization of who it is that he wants to bed; who he’s probably in love with. He makes it okay in his mind in two ways: one, he clarifies. He makes Damen say, “I’m leaving tomorrow”, and two. He’s so cold, so ruthless that first time.
k. Except when Damen comes, their eyes lock. And I’m pretty sure that what Laurent sees is how much Damen loves him. And he caves. He keeps emphasizing, this is over in the morning. You can practically feel how he gives in, and wholly embraces their time together, because in the morning, he’ll put an end to it, and Damen will go
l. Then, his uncle shocks him, and he asks Damen to stay for just three more days. But it’s just so that Damen can help him come up with a final plan. And they don’t touch.

ALL OF THIS TO SAY. That I think Laurent had very careful ways he thought of Damen throughout the first two books. He doesn’t ever use his name. He refers to him by classifications of his strength/his worth to Laurent. He gives in (because he’s in love) at the end of book two, but I think he truly believes that it will be the only time, and he makes that exception because he can’t say no to Damen and the way Damen is looking and him and because he’s exhausted and drained. (and because I think part of him wants to erase his uncle’s hold on his body and he trusts Damen to do that)

buutt i did just read that a third time and see "falling through scattered thoughts into sleep" UGH I DON'T KNOW.

I'm going to choose to believe that this dialogue exchange is so close that Laurent can't be confused yet instead.

[Damen took a step back, said: ‘Now you can say you were served by the King of Akielos.’
‘I could say that anyway.’
Lamp-lit, the room was filled with orange light, revealing its simple furnishings, the low chairs, the wall table with its bowl of fresh-picked fruit. Laurent was a different presence in his white undershirt. They were gazing at each other. Behind Laurent, the light concentrated on the bed, where oil flamed in a low, burnished container, and illumination fell on tumbled pillows, and the carved marble base of the bed.
‘I miss you,’ said Laurent. ‘I miss our conversations.”]

OH LOOK I'M BACK

Okay, Kindle highlighting that is evidence:
I found the exact line I'd highlighted in book one:
"Laurent had stopped dead the moment he had seen Damen, his face turning white as though in reaction to a slap, or an insult."

And book two:
(laurent finally apologizing for being a dick) ["but I would not treat you that way again. I was angry. Angry, that isn’t the word.’]

[‘I was never a fighter,’ said Laurent. ‘That was Auguste. But after Marlas, I was obsessed with . . .’ Laurent stopped. Damen could see the moment when Laurent decided to continue. It was deliberate, his eyes meeting Damen’s, his tone subtly changed.]

[‘That’s right, I’m still captured,’ said Damen. ‘Your eyes say, “For now,”’ Laurent said. ‘Your eyes have always said, “For now.”’]
I thought this was interesting. It was probably pretty obvious to Laurent that he was a prince

["I had the servants lay out some clothing for you that befits your new station. Very Veretian, you’ll hate it."]
Right before they had sex, Laurent even had him COSTUMED so it was less obvious who he was

[‘You’re not yourself,’ said Damen. ‘And even if you were, you don’t do anything without a dozen motives.’ Laurent went very still, the soft words half bitter. ‘Don’t I? I must want something.’]
THERE IS NO MOTIVE. HE HAS NOTHING TO GAIN. HE JUST WANTS YOU

[I wish I could have seen Damianos when Kastor’s hire-swords came for him...His father got exactly what he deserved, to die like the animal he was, and there was nothing any of them could do to stop it happening. Then again,’ said Laurent, ‘maybe if Theomedes had kept his cock in his wife instead of sticking it in his mistress—’]
this is when Laurent just learned about nicaise (sp??) and he's trying so so hard to push Damen away; to hurt him, and this is what he chooses

[‘You’d let me go,’ said Damen. This time it was Laurent who was silent, gazing back at him. Damen said, ‘And—until then?’ ‘Until then, you are my slave, and I am your Prince, and that is how it is between us.’]

I got out of order in pasting, but also this. Laurent is emphasizing their roles!!

MOAR LINES I JUST LIKED:

]Damen couldn’t help his amused breath of reaction to that, or the long, scrolling look from Laurent’s head to his toes and back again, which was probably a little insulting. But really. Laurent flushed. The colour hit his cheeks hard, and a muscle tightened in his jaw as whatever he felt was forcibly repressed. It was not like any reaction that Damen had ever seen from him before, and he couldn’t resist pushing it a little further.]
this just seemed so flirty and delightful


[Damen saw him, drawn out of one of the mostly intact tents not six steps away, and gone still at the sight of Damen. He was not wearing the woollen cap. His newly minted hair was uncovered, and he looked as fresh as he had emerging from the baths the night before, as he had waking beneath Damen’s hands. But he had resumed the cool restraint, his jacket laced, his expression disagreeable from the haughty profile to the intolerant blue eyes.
‘You’re alive,’ Damen said, and the words came out on a rush of relief that made him feel weak. ‘I’m alive,’ said Laurent. They were gazing at one another.
‘I wasn’t sure you’d come back.’
‘I came back,’ said Damen.]
SWOON

[Damen watched Laurent go, and only with half his mind experienced the urge to throw things. He knew by now that Laurent never acted precipitously, but always walked away and gave himself time and space alone to think. It was now time to step back and hope.]
you're learning!!!

[It was beyond Damen why anyone thought they were fucking. No one who knew Laurent would ever think that.
Damen drained the cup.]
i died. oh Damen

[Laurent, beside him, was percipient. Acquiring the fallen man’s short clan sword, Laurent inserted himself at Damen’s left, which, Damen noted without surprise, let Damen do all the heavy fighting. Until the moment when a clansman attacked from the left, and Damen, bracing himself to call hard on the muscles of his bruised side, found that Laurent was there, meeting the man’s blade, dispatching him with efficient grace, and shoring up Damen’s weak side. Damen, disconcerted, let him.]
<3 <3

[After a slight pause, Laurent said, almost carefully, ‘I realise that in my service you do not have a great deal of opportunity to pursue the usual—avenues for release. If you need to avail yourself of the coupling fire—’
‘No,’ said Damen. ‘I don’t want a woman.’]
This exchange was so tension filled and JUST ABOUT KILLED ME.

[‘This is not the way I planned to spend the eve of war,’ said Laurent. ‘With me in your bed?’ ‘And in my confidences,’ said Laurent.]

[Damen flushed slowly, heat spreading from his chest over his skin, his heartbeats intrusive. He tried to answer in a normal voice. ‘I can’t imagine you’d wear it.’
‘To keep. I wouldn’t wear it,’ said Laurent, ‘though I don’t believe your imagination is having any difficulty with the idea.’]
JUST YOU BOTH WAIT

I think I'm done now (except I'm starting book 3)