I'm not caught up on this thread, so please forgive if someone else has posted this as a potential theory: what if Casey gave Caylee to her brother, and he and his girlfriend are the ones hiding her?
Mind you, I have *not* gone through the various videos, transcripts, and info about him and checked them in the light of this idea to see what might fit and what would not.
But. We do know that he and Casey are very close. What if he is on Casey's side with regard to whatever conflicts there may be/may have been between Casey and her parents? Casey is kicked out of their home and takes Caylee with her ... she has no place to stay except bumming off her boyfriend and friends, and although all of them no doubt love Caylee, they are a party crowd and none of them would especially want a baby around 24/7. Lee knows this, he reasonably offers to take Caylee until Casey gets settled, and agrees with her to keep Caylee's whereabouts a secret from their parents.
Also, if she admitted to him about her stealing, he may have told her "Look, you are gonna get caught, what if you go to jail?" and she may have said, "I still want you to keep her. No way is anyone else to even know where she is. As long as I know she's okay, I don't care what happens." As far as this goes, it's possible her parents flat out told her that if she didn't pay back the money stolen from her grandparents, they would make sure charges were filed -- so Casey may have left that house knowing that chances were good she might end up in jail on those charges. We don't know exactly what all she has been involved in, so possibly she has friends who have been in jail, been to prison, and maybe she can deal with doing some prison time, maybe that goes with her "tuff" persona. Maybe she won't actually get prison time from all these charges -- she may get some jail time and the rest suspended pending good behavior for the next 3 years. Even if she produces Caylee, she'll still have these charges to face, so producing Caylee doesn't do anything to help her with them.
If the above or something like it is true, then I would think Caylee would be produced if Casey was charged with Caylee's murder. But it's not yet clear that she is going to be charged. Casey may be willing to play it out as far as it will go and wait to see what happens. Because at this point, once she does produce Caylee, there is a huge risk that she will lose custody of Caylee either to her parents, or to the child services system. She wouldn't want either of those to happen. Then again, maybe she would not lose custody, because if Caylee is safely with her brother or friends of his (for the time being), Casey is not being negligent about her care.
Something like this may explain the "high 5" between Casey and Lee.
The thought has been that if Caylee is alive and Casey knows where she is, then why would Casey allow all the searching to go on, the money to be spent on that, all the worry and upset people are feeling, etc. How about this ... (in her thinking) "Mom, Dad ... I told you I know where she is and that she is okay. You chose not to believe me. Fine. YOU set all this stuff in motion, not me. It's all on your head, if all these people want to believe she's dead and go looking for her, that's their business, I didn't ask them to do it."
Mind you, I'm personally mostly persuaded that Caylee is dead. But like so many, I really want to believe she is alive. If she is, then the above or something very close to it might be what is going on. It's less of a shot in the dark than believing Casey's story about the nanny.