Sorry I haven’t gotten you a response. I’ve been absolutely slammed trying to get my car ready.
Yes, if you’ve modified the throttle body, you’re now in CAM-C.
You can change your groups any time you want. However, the times you get only count towards points for that respective class. So, if you run Novice for 3 events, then switch to normal CAM-C, the events you did in Novice will not count points towards a CAM-C trophy (and vise-versa).
Your understanding of how you’re compared against competitors is correct. All novice times are compared against other novices.