Allying with an Exalt who is against Britannia is a good idea. Allying with an Exalt who against Britannia after pissing them off is not. And C.C. is not stupid.
If you've done something sufficient to piss him off enough to be an enemy, sure. But giving Nunnally a Geass wouldn't really be enough to do that, I'd think. Especially if it gave her the ability to see again. Lelouch isn't some kind of irrational psychopath - even if he's angry, he's not going to immediately go to the solution of eternally imprisoning C.C. unless she does something really bad.
This seems a little reaching. Unless she's playing an active role in their plans, I doubt they'd care- she's not the only Code Bearer around,
The Siddies care enough that C.C. apparently has to file paperwork to allow for a replacement. They don't likely care if she's being experimented on for a few years, but if she's out of contact for decades or centuries I wouldn't be surprised if they decided to send someone to check up on her. They might not, I just find it unlikely they wouldn't given that she's a person of interest to them.
and compared to Exalts (even DB's), Geass users are pretty weak and overly specific in their capabilities.
I wouldn't underestimate Geass users. Even if their abilities are specific and the users themselves are mortal, the actual abilities can be very strong and with few exceptions Geass can be spammed.
Lelouch definitely would look into Geass thoroughly if Nunally contracted. He'd check the libraries of Hell, ask demons, and so on- nothing is going too far for his sister. And as you pointed out, C.C. would know that. And whilst she might not recognise his specific Exalt type, she'd be an idiot if she underestimated the ability of Exalts in general to find stuff out.
And what would he find out that would be so dangerous to her?
Lelouch: "You want to make my sister immortal?"
C.C.: "That is what would happen, so yes."
Lelouch: "Well then, carry on."
Once Lelouch finds out that his potential lifespan is vastly extended, knowing that he could make his sister live longer would probably appeal to him.
Being crippled means she can't go around using her Geass easily. And if C.C. starts trundling her around so she can use it, you'd better believe that Lelouch would take a very close interest in her.
Unless C.C. starts forcibly taking Nunnally out, then again I suspect that Lelouch will begrudgingly accept his sister's decisions. He would take greater interest in what's going on with Nunnally and C.C., but as long as C.C. isn't doing anything to hurt Nunnally then she wouldn't have to worry too much. And hell, just like in canon she would refuse to take Nunnally anywhere if she thought it was too dangerous - remember when she tried to stop Lelouch from going out at gunpoint?
Also, you still haven't explained why C.C. would pick Nunnally over the far easier and more plentiful targets she could find wandering around a Ghetto or something.
Basically, whilst I agree it's possible that C.C. might target Nunnally for a contract, I find it rather unlikely given the risks and complications involved and how easily she could avoid those simply by picking someone else. And there are plenty of ways to prevent it (Demonic Spiders lurking intangibly around Nunnally at all times, for example).
Random targets in the ghetto who have nothing in the way of resources and whose personalities she doesn't know much about? She's not going to choose someone randomly, especially after Mao. Nunnally isn't necessarily a 100% ideal target, but I imagine she will come to mind as a strong candidate for consideration.
Also, we don't have enough reason IC to have Lelouch try to stop C.C. from making a contract by having the Anuhles stop her. He knows C.C. is alive, but that's it. He's got no reason to suspect she knows who he is, much less her having any interest in him or his sister. The orders the Anuhles are likely to receive are "kill anyone who tries to hurt my sister" and if C.C. is just offering power they probably won't interfere with it - hell, in their minds getting power would probably be a good thing.
Anywho, we should probably stop as we're likely to keep going in circles. We'll just have to wait an see what happens.
Of course, C.C. could just show up without contracting anyone, and just offer to help Lelouch out if he'll shelter her until she can make other arrangements. An Exalt of his level is bound to attract some interesting candidates.
....that would be sweet lol
