First, I think saying you are unaware of why she was on that road would be a fairer assessment than "There could be only one reason for her to be there"
I haven't took a look at the road myself, and I will, but I am not sure how you could rule out a drug deal. A drug deal can theoretically happen anywhere, and typically a place you don't expect it might be a good place to do one. ya know?
In terms of why I don't find the schizophrenia theory as likely. it's because I know several people who have schizophrenia. Two of them had it develop in their early 20's, one of them in their late teens. I know some people believe that it is triggered by drug use, and in my own personal experience with two of those individuals that seemed to be the case. one wanted to do shrooms with her boyfriend and then after that one time, she had full blown schizophrenia symptoms. She developed paranoia that increased as time went on. It was not in any way subtle imo.
The teenager didn't have any drug trigger that I know of, but the condition ran in his family. His paranoia started out more subtle with things such as thinking people were poisoning him at restaurants. But, if you were around him, you wouldn't right away know he was schizophrenic, unless you engaged in conversation, then it was very clear quickly that something was wrong as he might become highly uncomfortable and say odd things. But he wasn't like the other friend who was more active in engaging in her paranoid delusions. She'd seek you out to tell you things. The other, again, emerged in her first drug use. She had people that weren't there talking to her constantly and would talk back to them. She often thought I was her brother for some reason and would constantly tell me about celebrities watching her.
My point here is that none of these were subtle enough that I don't think a police officer or SOMEBODY around Toni would be very aware of this type of behavior. When the people I spoke of were on their medication they often became zombie like in some ways.
Not saying that this is the only way a schizophrenic person can act, but I've never known a schizophrenic where it was subtle on or off medication. Even if on medication you are gonna know something is really wrong. So why wouldn't the officer notice this? I can only imagine how any of the people I just described would have responded to a police officer, my guess would be extreme paranoia if during a psychotic break. I just have trouble believing the officer ain't gonna notice. If she was on medication, the officer would notice imo, because that is soooo noticeable imo. However, I am just basing that on the several people I know with the affliction, and they were not allowed to drive anymore. Maybe there are more highly functional schizophrenics.
I went to the movies with my female friend with schizophrenia, I have known her since high school. We once went to see the movie The Watchmen and she was excited about seeing it, she was on her meds at the time and seeming much better than previous. We'd went to the movies before, never had an issue. I didn't know much about schizophrenia at the time, and now I will just say... don't go to the movies with a schizophrenic, especially a movie where you have an enormous blue dude walking around blowing people up. Things got real very quickly when that blue dude got down to business. I don't know that I could even explain to you what happened and make it as scary as it was to me going through the process of leaving that theater. But no... not at all subtle.
The above is why I am personally skeptical, however that Toni was schizophrenic and no one noticed it or mentioned it. I also find it hard to believe a officer wouldn't have noticed it. They must surely run into this type of thing semi-often. I also wouldn't be surprised if an officer might trigger some kind of paranoia that would become immediately noticeable.
Now... maybe if she had her first psychotic break after the pullover? Sure. But just doesn't seem likely imo. I'd be more prone to believe she took acid or something at the park for some reason as someone else suggested and then had hallucinations or triggered her first psychotic break. But that's just me theorizing for the sake of what'd be the most likely scenario that involves a psychotic break. Now if someone around her stated that she was showing signs of schizophrenia, I'd be a step closer to seeing where you are coming from. I'd also imagine working in her occupation might be challenging if she was schizophrenic. But again, I am just purely basing it on my personal experience and what I know about it, I certainly am not an expert on the topic. There could be a far wider range of behavior than I have witnessed, that might fit your theory better.