Jillio-
Have you wondered why it took them 7 months? If it was that serious, then it would have moved swifter. Remember, to obtain an arrest warrant, there has to be a PROBABLE cause that a crime occurred. As definied in the wiki, ""Probable cause" is a stronger standard of evidence than a reasonable suspicion, but weaker than what is required to secure a criminal conviction. Even hearsay can supply probable cause if it is from a reliable source or is supported by other evidence."
With that definition, it is easier to obtain arrest warrant, but harder to secure criminal conviction. Which means there will need to be more evidence in order to convict her.
Another thing to keep in mind. If they thought that she posted danger to the community, with the potential chance of sexually assaulting someone else, wouldn't she be detained sooner? It is probably because they had MINIMAL evidence, and not very much evidence to secure her arrest 7 months ago.
The police also likes to charge a person with the HIGHEST possible charge because majority of cases tend to be pleaded down to lesser charge to get conviction. For example She may be accused of sexual assault, but the evidence may point to more of a sexual misconduct which carries less punishment. It is all strategy of the legal system.