Doing my own investigation work:
Islands of Adventure closed at 8pm on Sunday.
The way this story is painted out it seems that the woman would have had to been in a common area in the bathroom. The alternative scenario that she was in a stall, and someone shoved a knife under the stall, cut her, then asked for money and while she was getting the money, cut her again seems STUPIDLY UNLIKELY.
Although, slightly slower around 5pm I don't think any of the bathrooms at Islands of Adventure would be empty at this time.
The only bathrooms that would be slow enough for this crime to happen in our the ones outside of the gates (ruled out, would have said Citywalk), bathrooms in Marvel across from Icecream (more than likely too busy), Toon Bathrooms by Popeye Exit (still calling too busy), Toon Bathrooms by Ampitheater (slower bathrooms, area too busy), bathrooms by Sinbad (someone would have seen perpatrator, area too dead), Suess bathrooms closest to Lost Continent (too busy for crime).
If it happened in a restroom common area more than likely there would have been a witness, or 2nd victim. If it happened in a slower area then it would have been more likely they would have seen the perpetrator.
Flat out, at 5pm on a Sunday, the parks are too busy for this crime to have happened. If the bathroom was slow, which it would have had to have been for it to be in a restroom common area, then they're going to be able to pull up the cameras and see that she was either A. The only one leaving the restroom or B. The perpetrator.
While SLIGHTLY POSSIBLE, Jake says MYTH BUSTER