I wouldn't count on her being able to spy on Kaname. He should be able to encrypt things fairly well by now. Most of the stuff he 'privates' isn't very relevant to her interests anyway. He's kind of a weirdo like that.
Cornelia. You are free to pop into those. At least, for now. She's still learning about the network, and she definitely isn't a genius like her brother. But she also is wary of Lelouch, and so she's been aiming not to have any truly important conversations over the network, even if they are private. If she doesn't want her brother to hear it, she'll probably meet with the other party in person.
And fffff... Akane rarely uses private filters, but LOL... she haz no ninja skills. If, for any reason, you want to bust into one of her conversations and laugh at the tsundere~ness, you have my permission. XD