Something's not right about this whole story.
In a series of extremely detailed text messages with her boyfriend that span a time period of several weeks...
Unless you "happened" across these by accident, there was a reason you were checking her phone.
This kid has a good homelife, doesn't do drugs or drink...
I would be very surprised if this is true and I don't see how you could possibly know this. She's not on any medication at all? If so, then something probably happened in her past. I would be shocked if she's not on meds or something traumatic didn't happen in her past.
And dchuk's suggestion is pure idiocy. A person on anti-psychotics, relaxants, uppers/serotonin enhancers has a greater chance of committing violence/suicide than someone not on them, especially if they're already toeing the line of sanity.
I would simply confront her and see what the reaction is. Even if her response is to lie and she thinks that you're buying her lies, that in itself is telling and means she could have sociopathic tendencies. But it's probably too late for this.
I wouldn't have gotten the "mental health authorities" involved, whoever that is. They're only gonna drug her up and completely push her over the edge into insanity. The "mental health" profession is an utter joke, in my opinion. They ruin people who could otherwise be saved.
Based upon her reaction, I would then either drop it or bring the hammer down, i.e. see what legal consequences there are for conspiring/plotting/planning someone's murder, using the texts as evidence. I don't know what the laws are in that area, if she would go to juvie, etc.., something to shock her world and let her know she's drifting into Crazytown. Again, it's probably too late for this since you've apparently gotten someone involved.
It could be that she misunderstands a lot of things about how you feel towards her, perhaps is jealous of her mom's attention towards you, perhaps resents the fact that you took the place of her father, maybe is slightly paranoid and thinks the family has frozen her out, and sometimes getting all that stuff out in the open is enough to defuse that built up tension. I don't know how you can't possibly know her reasoning if you saw her texts, she must have mentioned some type of catalyst for her psychosis.
This is why I would never get with a girl with kids, much less marry them. You should probably ask your GF if anything happened in the past.