No, I don't think she will take the immunity deal if it means she will have to disclose the whereabouts of Caylee's body. Even if Casey did murder Caylee she has been given every opportunity to admit to an accidental killing. It seems she would take this route out of her situation. It is, after all, a much more believable defense than Zanny the Nanny. But she won't admit to it being accidental (even if it, in fact, was) because the body displays some type of evidence of a horrendous act on Casey's part (possibly with help). Perhaps the body shows evidence of murder or, if accidental, shows a horrible attempt at hiding the fact, i.e. she tried to burn the body and it didn't burn so well. If so, she wouldn't be able to admit to it being an accident because she wouldn't want the body found in a charred state.
Just my opinion but it is the only explanation I can think of as to why she won't just say it was an accident if it was.
At any rate, she doesn't want that body to be found whether it was accidental or otherwise.