Ok this is just to wrap my head around some things because I am so not as awesome as Elder Sister. This is so not revised or spell-checked or anything, but it makes me feel a little better.
RECAP:
As I've said before, Lelouch from the start sees Cornelia as his primary opposition. He's no doubt heard of her immediate and brutal methods for handling the situation with the rebels. His opinion of her as ruthless and willing to go through any lengths to, as he sees it, win isn't helped by Cornelia's slaughtering the civilians in the Saitama Ghetto just to lure him out. He equates the scenario with Clovis' destruction of Shinjuku for the sake of keeping his failures (losing C.C.) secret. But he quickly learns that the quick and easy way he handled Cornelia is less likely to work with his older sister, as she swiftly hands his ass to him in battle, taking out his troops and nearly forcing him into a stalemate. This blatant misjudgment of her skill leads me to believe that Lelouch's interactions with Cornelia when he was growing up were quite limited, despite his attachment to Euphie. Even his cockiness can't attribute for such a move. The experience is a sobering one for him, for the first time leading him to understand his vengeance cannot be carried out on his own and showing him that handling Cornelia will take a greater degree of finesse. Said finesse comes into play during the hostage situation where he uses what he does know about Cornelia (her closeness to Euphie) to manipulate her, partially getting back at her for Saitama and partially using the incident to throw the spotlight on his new army and image.
There is much antagonism between the siblings on Lelouch's side because of Cornelia's ability of countering Lelouch's challenges, making him more determined than ever to defeat her. There is also the issue about Lelouch seeing Cornelia as tying into the conflict set by his father, which champions the belief that the strong shall rule the weak. Cornelia represents this same violent mindset, which stands in opposition to Lelouch's hope to procure a peaceful world in which Nunnally will be safe. I truly believe that some part of Lelouch does want peace, he just believes it can only be achieved through the destruction of the main cause of conflict: Britannia. This also goes along with C.C.'s question of what matters more to Lelouch: Finding out who killed Marianne or Defeating Britannia. Since Lelouch's driving forces are vengeance and Nunnally, not to mention he believes his family is responsible for his mother's death, he sees them as one in the same.
That said, I do not believe Lelouch wishes to kill Cornelia, as what happened with Clovis does affect him and was largely the result of blood lust (created by the deaths around him, his own receipt of power he couldn't understand and the stunning possibility of his own death) and a need to convince himself of his cause. Lelouch is never again so quick to kill unless he is in direct danger. Again, I don't think Lelouch respected or liked Clovis at all, but I also think murdering his siblings is harder than he wants to admit when he's in his right mind. Lelouch has conviction, but he also has those incredibly strong attachments to the past. Will he use Cornelia? Hell yes because that's what he does.
WHERE THINGS STAND:
Back to camp, as I've said before, much of Lelouch's interactions with Cornelia are heavily influenced by frustration at his position/powerlessness. All of his greatest tools are beyond his reach so there's that nagging need to feel like he has the upper hand. He challenges constantly to shake this feeling, as we all know how bored with life he can be when he feels he can't do anything. It's selfish behavior that he does regret afterwards because he knows it complicates his plans of getting closer to Cornelia and learning things. But Cornelia in turn challenges back, both perpetuating the cycle and giving Lelouch an unconscious sense of amusement at having such a consistently strong opponent. It's something he enjoys in a manner that may not be as cynical as he likes to think.
Then we have the de-aging. This is probably the first time Lelouch has spent a significant amount around Cornelia as a child. During this time, there was a sense of protection and occasionally happiness at being around his elder sister. This is partially why Lelouch became so agitated when he heard other people telling him things that clashed with what Cornelia told him. Part of him did want to believe her. In the end, he mostly could. There was always a sense of respect contributing to his actually doing what she told him to. She is his elder sister after all, and he does see her as smart so her decisions had to have some merit. Lelouch remembers all this and isn't that greatly surprised at his sister protecting him in general -- he understands the instinctive reaction around children and there was also Euphie's feelings to consider. He is however edgy at being unable to understand why she would continue to let him have his lie in a place where it wouldn't matter and where there is no danger. It further confirms the notion that Cornelia actually does care about Nunnally and himself, something he has been trying to put aside because of his need to view things objectively.
A major piece of this is Euphie. It's important to note that Lelouch has not agreed to the SAZ; he has merely told Euphie he would help her refine the idea (i finally found this link to verify this). Lelouch probably doesn't think Euphie's idea will really be successful, but he doesn't worry about this here because he knows he actually enjoys his time with Euphie. He is willing to pretend a little for that. Lelouch will be Lelouch for her. The fact that Lelouch is still not sold on Euphie's plans comes up most often in his discussions with Suzaku because his stubborn self he thinks he truly has a chance of convincing Suzaku if he stays at him. Lelouch is also partly banking on the hope that Suzaku will in turn discourage Euphie. But knowing that Euphie is doing the SAZ for Nunnally causes him to have second thoughts as well.
That said, he knows Cornelia is neither as compromising or as idealistic as Euphie. She will continue to stand in the way of his plans. He cannot regard her the same way he regards Euphie in this respect; he has no sway over her. Cornelia is too devoted to Britannia. He sees this in her constant overbearing on the Japanese Suzaku, even though Suzaku has proven himself a loyal knight on countless occasions. Even if Cornelia understands why he has to do what he does, she will not condone it. She simply cannot. Not only that but Cornelia is responsible for the same atrocities as Clovis and he's committed himself to punishing those actions. Because of these lingering facts, Lelouch has tried to forget about Cornelia's likely concern for him in favor of viewing her as an obstacle. As their antagonistic relationship continued, this became much easier as Cornelia's familial side showed up less often.
Now again Lelouch is forced to remember that Cornelia is family, family that he almost became quite close to, family that acknowledged his older self's wishes. Part of him wants to convince himself Cornelia has an ulterior motive. That makes things easier to understand and doesn't interfere with the objective view he has steadied himself to take. This part is wary and unsure whether Cornelia is trying to manipulate him as well. Although she may have shown herself to have remnants of feelings for him and Nunnally, he knows she is also just as eager to gain the advantage. So he is uneasy. More uneasy because if those actions were sincere, it complicates things that much more. Lelouch knows he cannot have it both ways, and he's not naive enough to think that Cornelia can be eventually won over like a certain knight. As always, he must take the path he's set out for himself. But internal conflict is far more likely. Either way, he's probably going to be considerably more guarded for the time being.