If the jury can not agree on a guilty verdict, it only takes one person, then there will be a mistrial. Jury misconduct is another option, I believe.
I would hate to see a mistrial, because I am ready for this to be over...but, as said above the state usually obtains a conviction on the second trial, after a mistrial. I have no doubt the state will go for another trial should there be a mistrial.
I would be shocked if there was a mistrial (but, it wouldn't be the first time). From what I have seen Florida juries are likely to convict, when the evidence proves the crime. Many convictions are based on circumstantial evidence, and this case has an abundance of it. The huge amount of evidence, in this case, will secure Casey's conviction, I believe.
I don't see Casey's current lawyers staying on for a second trial...maybe she can get a public defender ?