The forensic extractions alone might not be enough because another thing that happens in these kinds of circumstances is that some things are deleted from the phone, the phone is backed up, a factory reset is done on the phone, then the phone is restored from the backup. This will result in the permanent loss of data from that phone. So if there were dozens of iMessage messages deleted there would only be the indication of that (gaps in the message IDs in the message database) but no way to identify who sent or received those messages without going to Apple as just one example. In that example situation they probably could not get the message contents from Apple but should be able to learn the identity of the other party/parties in the deleted conversations.
Another example would be where contacts are exported, certain ones are deleted from the exported file, the entire contact list on the phone is deleted, and then the modified exported contact list is imported. This leaves no gaps in Contact IDs. However, it does leave the time the contact was added as a clue - many contacts will have the same added timestamp and the next ones a second later and so on. Continuing with that example someone might upload their contacts from their phone to an e-mail account on Google or Yahoo! or even Facebook. In the case where contacts were cleaned up from the phone it may be possible to identify other contacts from those e-mail accounts or Facebook.
Similar things can be done with photos or video, uploading them to DropBox as another example.
The basic premise is that just because something has been deleted from the phone and is not recoverable from the forensic dump it doesn't mean that what was deleted doesn't exist elsewhere. It may not turn up, but it should be something that LE at least tries to find.
The reason LE should look at Yahoo! (and any other known e-mail providers for MB) is that when you create a new account you usually supply another e-mail account to receive the verification message or a link to access the account for the first time. Often times this verification e-mail will be deleted but Yahoo! might have the information that would identify that a message was sent to the known account from another e-mail provider's account that sends out such verification messages. In this way you can identify any previously unknown e-mail accounts. It is not uncommon that the newly created account elsewhere was not then subsequently used as an address in the creation of another account, an so on.
What I would be looking for in Missy's case is anyone that knows or should know more about what was going on with Missy because they were communicating on these kinds of secondary or hidden channels or if they were participants in deleted conversations. If they hadn't been spoken to yet they should be interviewed. If they had been spoken to and did not volunteer they knew more then they should be re-interviewed like, yesterday! It may be the case that they don't know more but LE should find that out.
