2) can't apply because any competence or lack thereof did not come into play, due to aforementioned sanctuary laws.
Does apply , since he was arrested and processed , you cannot process someone unless you can establish their identity , so either they were incompetent in accepting a fasle identity from a criminal or they accepted a true identity and didn't spot the outstanding warrant .
Sanctuary laws don't even come into it . They would come into it if he was only stopped on suspicion of drink driving and let go after a roadside check showed he was not drink driving , but not once he is arrested .