EMOTIONS
The best way to explain an INTJs emotions is a quote to Star Trek's Spock by his father , Sarek:
"Emotions run deep within our race. In many ways more deeply than in humans. Logic offers a serenity humans seldom experience. The control of feelings so that they do not control you."
It's not that we are without feeling; we are very controlling if the emotions we display. Alcohol can give us an emotional BM (upvote for emotional BM).
Volatile and in Need of Conflict
In my early relationship with my wife, when we argued about something and she failed to see the logic of it, I would get frustrated. Even more when she failed to engage in the conversation or had illogic. As an ISTP, she doesn't think to far ahead, doesn't know how she feels about something and an argument with her is name calling - still is and it's a big problem. I wanted to hash out of differences with logical debate, something she's not capable of in the moment. I want someone as quick as me. Eventually, I didn't press those issues like I used to unless it really mattered.
It's not typical of INTJs, but I know of a female INTJ becomes abusive when stressed. We tend to be expectatious (upvote for perfect made up word) and the immature among us will become abusive. They're not healthy to be around.
Social
At parties, I will talk with a wide range of people, like a social butterfly but eventually fleeting. Socializing can be strategic and it can also be a way of getting through a social event. If I make a one-on-one connection with someone and have genuine conversation, the event becomes easier. I perform with sincerity in my dealings with others. My social endurance sucks, but it's not fake. I'm genuinely interested in those I talk to and that can be folks I have nothing in common with. In fact, I have a bad habit of trying to include everyone. I have focused my attention on those that are uncomfortable.
Impulsive
Regarding the serious relationship, I would guess that if she felt it wasn't going to work out, she knew she could cut it off at any time. I have been eager to start relationships in the past knowing what I want. Meeting the parents to me doesn't mean anything to me. At the beginning of a relationship, I will lay everything out. No surprises. It might be why she told you of the cheating in the past. It doesn't necessarily mean she will cheat on you. However, I will say, it does say it will be easy for her.
Advice
She's not worth your time and she has a lot more maturing to do. You're only going get pain and grow bitter, which it already sounds like you are so short in the relationship.