I hope she is prosecuted
The messages clearly mean the prosecution cannot prove beyond reasonable doubt that Allan is guilty. That does not mean they can prove beyond reasonable doubt that she lied. It may be that anyone reading the messages would think it is highly probable that she lied but that is not the same as proving it to the required standard for a conviction.
Even if the messages do give the required level of proof, there is a reasonable chance that the CPS would decide that it is not in the public interest to prosecute on the basis that it might deter genuine rape victims from reporting the offence.
Surely every bit of evidence should be given to the defence
The legal requirement is that the prosecution must disclose any evidence that "might reasonably be considered capable of undermining the prosecution case or assisting the case for accused".
The police often collect a large amount of evidence when investigating a crime, much of which is completely irrelevant. What should happen is that the police should provide the prosecutor with a detailed schedule of all the material and the prosecutor then decides what should be passed to the defence.
Whilst this could be seen as unfair, most of the time this process works well. Most prosecutors don't want to win at any cost and will ensure that the defence has access to all relevant material. It is also possible for the defence to apply to the court for disclosure of any material that has been held back by the prosecution. If that happens the judge will decide whether or not the requested material should be disclosed.
Whilst the system does work well most of the time, it is, unfortunately, a common feature of miscarriages of justice that some important evidence that could have helped the defence goes missing. Our police are not perfect and, despite all the improvements that have been made, there are still officers who will plant evidence to convict someone they believe to be guilty and hide or destroy evidence that damages the prosecution case.