Sorry, I just re-read your post and realized the cop was off-duty too. My understanding of Canadian law is that all of you were legally considered to be first aiders (I could be wrong) and that, as the first on the scene, you were in charge until you chose to hand the scene over to (a) another first-aider, (b) EMS (on-duty), or (c) qualified medical personnel (e.g. a doctor or nurse).

If you were the highest qualified person on scene, you did not have to relinquish the scene to either of the other two. If they ignored your advice and risked injury to the casualties, knowing that you were a paramedic with several years experience, I think they should both have been reported to their respective superiors. The police officer probably would/should have been subject to disciplinary action, the other paramedic could have his certification pulled. If he didn't have any certification, he might well have been subject to legal action.

I don't see how your certification would have been in danger if you stayed, as long as you had clearly stated your qualifications and that you were in charge in front of witnesses. Any subsequent investigation would reveal that any mistakes made were made in spite of your actions, not because of them.

My biggest concern would be that the police officer and the other paramedic will probably not have their behaviour corrected unless someone makes a formal complaint. By leaving, you legally seem to have agreed that the casualties were in competent hands.

Again, this is based on what little I know of Canadian law; your laws may well be completely different. However, having gotten another paramedic to state, for the record, that he was now in charge, was technically the correct thing to do but may not have been in the best interests of the casualties.