So... I don't know where you read about the parental puchase thing, but the government of Virginia seems to disagree.
The NIH says so: ""making a delivery of alcoholic beverages by order of his parent". To me this reads like you're allowed to carry alcohol in Virginia if your parents told you to go and get some, no matter how old you are.
Possession, not purchase. Purchase is specifically prohibited, also in your NIH link. The parents would have to purchase it for them (or obtain it somehow), and then send them on their merry way. I assume it's for "take this wine to Grandma's"-esque situations. Since the agents were waiting around for whatever reason, it was pretty obvious to them parent/guardian wasn't there to give it to her. Since the water looked like alcohol to the agents and she looked like she was likely underage (and from the other article, might seem overly jumpy and alert), they wanted to ask her about it. That's completely within their purview. There is very little suspicion required to question somebody without detaining them.
If your law enforcement has "SS" tattooed on their arms in Arian runes, yes. Otherwise no. Most people, police included, forget what the police is about and what their duty is. Their duty is to serve and protect the citizens (US police even have this written on their badges). Yes, this includes making sure that people follow the law. After all, the law, too, is to serve the people.
they wouldn't be enforcing the law, which is the raison d'etre for law enforcement
However, their duty is not to harrass and oppress people, nor to suspect and "criminalize" people when according to the situation, it is a perfectly reasonable assumption that they're not doing anything wrong.
A) Godwin's Law
B) Law Enforcement
C) Walking up and asking a question isn't oppression or harassment, and their job is to investigate probable crimes (so they can stop them if they are) insofar as it doesn't violate your rights. More about that. Again, you have the right to not answer any questions and leave in a non-suspicious way (ie, not running and/or look like you're destroying contraband or grabbing a weapon, etc.).
The key issue in this encounter is that the girl responded in a suspicious way (though she did so for entirely understandable reasons), and the agents reacted to that, which only freaked her out more, and so on.
The thing I'm not sure about it is why she started her car, my car lets you turn on the AC/powered windows without actually starting the engine. She might have just been jumpy, though.