Would only like to comment on these specific 3 quotes:
We will never know until the ending about how will those characters change. Ran may well start being involved in the Black Organization story later, at the final stage and, we don't know how good she would be in fighting alongside Shinichi. Maybe, better than people think.
Also, who knows, it's not a sure thing he will start being focused on global mafias, maybe he WILL return to investigate "normal" murders. A life of a global anti-mafia fighter is tough, we don't know whether he will make such an extreme decision, as to dedicate HIS WHOLE LIFE to working with the FBI.
Also, since Shinichi wouldn't probably be the same after the story ends, as you say, how do we know he won't change for the better due to the experience and won't start appreciating other things and people (Ran including) more than before, since he will be more experienced and serious now, not just a somehow careless teenager?