For starters, drivers licenses can be years old. So using outdated photos to eliminate possible suspects for a crime committed 4 days ago seems like a great way to let the perp fall through your fingers. I know that's no great loss in your mind, but that's more of a legal debate. Today the police are still charged with pursuing criminals to the fullest extent the law allows. And a broken tail light is an offense they can legally stop you for (I should know, its happened to me).
What do you mean "2) the person is not actually the suspect and he registered the car under his name, and his driver's license can be compared to the suspect's, and lack of identification can be confirmed. (And no engagement is necessary.)"
Do you think the police already know the IDs of the perps?
And I don't presume Castile was the perp. I do think its impossible to 100% rule him out, based on the information we're aware of, and stopping him seemed justified, because like a lot of people, I want the police pursuing armed criminals.