The FAA has does not employ enough people to check who ran a drone up and down a river for a half hour on a given day. They are not like fish and game wardens who check licenses at boat put-in points. But even if one did happen to come along while you were involved in saving a life, do you think that such a person would want to bring shame on his employer by citing a rescuer? Not if he likes his job he won't! The whole anxiety about the FAA as patrolling drone police seems to me to be misplaced.
I would be very open to hearing other points of view with other information.
I think you are mostly correct, but it's not the random, patrolling FAA agent that you need to consider - it's what happens if someone other than the LE officer who asked you to help reported you for flying illegally - for example if other aerial resources did show up unexpectedly.