Well there are two ways to provide evidence for Kallen's defection.
One is to assume she WILL defect and look for evidence to support it. The OP and ED provide plenty, plenty, of hints to confirm this suspicion. This is, of course, if you take the defection as fact before its proven.
The other way is to think of reasons or methods that might be used to cause her to defect. A possibly budding to loyalty to Nunnaly, a possible understanding for Suzaku's methods after a discussion with him which will occur next episode, and Suzaku telling her about what happened regarding Euphie all might send her packing. Its possible that she will forget avenging her brother if her hatred for Lelouch is stroked enough. Her low intelligence rating does not help her either.
I am personally a pessimist. I can sit and point out the numerous ways things can all fall apart with my favorite pairing all day long, but it is vastly harder for me to come up with reasons why it would survive. Its possible, but I can't really see it happening.