I'm trying to do some segment mileage runs on USAir for Air New Zealand
airpoints.

These involve using discount first class fares.

If on some of the segments there is no first class on the plane, is that
segment treated as first class anyhow or coach?

If on some of the segments there is no first class on the plane, is that segment treated as first class anyhow or coach?

You'll probably want to ask what the booking class is for those segments - it's possible that the RJ segments may default into Y codes, with the others at A, which would change the preferred miles on US. You should be able to view the rules online when booking via US, or ask the phone agent what fare class they're booked in.