What's the meaning and motivation of Izaya's behavior in Episode 2: "Highly Unpredictable"?
I think you're over-thinking this.

Izaya, for lack of a better word, is a troll. Period.


  One who posts a deliberately provocative message to a newsgroup or
  message board with the intention of causing maximum disruption and
  argument.
  (taken from Urban Dictionary)


But for real-life personal interaction instead of on the internet.

In fact, he is an amazing troll - as evidenced by his ability to read people's minds and predict their actions. Which in turn gives him a great ability to "toy" with people how ever he pleases.

For pretty much the entire show his actions are always to provoke responses in other people without regard to their well-being (or rather to intentionally hurt them in specific ways).




  he is being somewhat inconsistent when telling her about all the people who jumped from the roof. At first, it seems that his actions are directed toward helping Kimichika by showing her that her case is neither special nor too bad, and it is not worth to jump.


Since he admits to trying with people to see their responses, this type of inconsistency could very well be intentional to provoke a reaction from Kamichika.


  When Izaya sees Celty helping the girl, he doesn't act surprised at
  all, much as if he was expecting this scenario.


That's most likely because he knows Celty pretty well and it's common for her stick around and save people.


  Kimichika will jump nor that Celty will save her (I think he didn't
  even know that Celty didn't leave)


As a troll, it probably doesn't matter to him. But it's very possible that he knew Celty will save her and he wanted to see Kimichika's reaction after thinking she would die only to be saved.