My guess is that you like hip hop? Here's my best at it.
The phrases are synonymous. But remember a "grill" is what you barbecue food on. So saying, "Why you all up in my grill," may literally mean, "Why are you all over my food that I am grilling," or like you said, "Why are you meddling in my affairs."
Regarding being in someone's face, it means confrontation. When people fight, they usually face off. Part of the body language people use to show they want to fight is to put their face right in front of the other person's face.
With the above said, you might already know the answer to your other question about "Why you all up in my grill when you haven’t been invited to the barbeque?" This might be taken literally as, "Why are you tying to eat my food, when I didn't invite you to the barbecue," or "Why you messing in my personal affairs when you were not invited to my party?"
And about "Spit game..." I think "spit" means to "talk," and "game" is the ability to woo (or win) girls. Therefore, to "Spit game" means to flirt with a girl or try to win her affection--but by saying how much you are the man.
Finally, "Nerve racking" is actually, "Nerve-wrecking" and is anything that makes you nervous. To "kick in" means "to begin to have an effect." So, “Approaching females can be a nerve racking task, especially when insecurity began to kick in," means "Approaching girls can make a guy very nervous, especially when the guy begins to feel insecure."
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