The state has indicted Casey on particular charges. The defense team doesn't have to offer up any other theory other than saying their client is innocent and forcing the state to prove their case. However, in the course of defending their client against the case the state might make, the defense could pose questions such as "And could an accidental drowning have caused Caylee's death?" or "Is it possible that Caylee got hold of some medication and accidentally poisoned herself?" and leave it to the state's witness to prove it couldn't have happened that way. Casey would never need to take the stand in such a situation. Just saying "could it have...." is just asking the State's witness to admit there could be a cause of death other than deliberate homicide.
It is ONLY if the state is going to go with a firm different story, such as "ZFG took her at JBP to teach Casey a lesson" or coming up with a firm accident scenario that Casey would be compelled to take the stand to answer the state as to why she told the original story, both in written sworn statements and then in a very long verbal statement to LE.