Hookpunch, good work stopping and rendering assistance. You were right to look for guidance in treatment from the others arriving on scene. Don't let any outcome for the victims get you down, learn from it and move on.

As for the rest of this, I'll talk out of my alternate orifice:
- the arriving off duty fire fighter did the right thing by evaluating *all* accident victims (the two in the other car), not just the victim who was inverted that you were directed to. That alone would count for some points with me in determining who was in charge of the accident scene. Its always a risk that folks who are early on scene will tunnel vision on one victim and ignore some others.
- cutting out the inverted victim: that's a judgment call imho, maybe a paramedic could clarify the protocol for an off duty doing this. I couldn't make that judgment unless there was a proximate risk to the victim (fire), or I detected they had stopped breathing (ABC). If the off duty firefighter made the call before any other help arrived I would assume it was because he assessed the victim was in danger due to the inverted position - I might directly ask, should we wait for EMS and more equipment, they are minutes out. He might stick to his initial assessment, or go with the assessment of the 911 operator to stay put and stabilize the victim. If he decides to cut the victim out, you do your best to assist with stablization during and after cut out.
- LEO made his own decision to cut the victim out: assist with stablization during and after the cut out.

The rest is noise, you were okay to be confused by it. Some responders might want you to continue assistance if you are immediately involved and they don't want you to move off task (my first accident victim I held her head in place for 20 minutes until they could get a collar around her), also with additional responders they may have re-assessed the other two victims and been short of hands for a while, which subsequently showed up, putting you more in the way than being any help. Don't sweat it - you stopped, you kept your assistance to your training, almost any responder should thank you for that.