Question: I noticed a 48-star flag on Danny Reagan's desk. Why is it there?
Question: Just what is it with them always relying on each other in the police department or the D.A.'s office? They don't trust anyone else?
Question: What is the pin Mayor Dutton is wearing?
Answer: NYPD police pin.
Question: What pin does Jamie get at the very end?
Answer: This is an NYPD Meritorious Police Duty bar, specifically either the Honorable Mention (silver-star) medal, or the Commendation (bronze-star) medal. Recipients are awarded Honorable Mention for "an act of extraordinary bravery intelligently performed in the line of duty at imminent and personal danger to life." And the Commendation is for "grave personal danger in the performance of duty" or "highly creditable unusual police accomplishment."
Question: I am wondering what style of hat Danny Reagan wears in the show?
Question: What brand of cigar does Frank smoke? They look like Fuente.
Answer: On the old episodes, it looks like God of fire, but the newer episodes look like a box of Opus X.
Question: I thought Danny's wife Linda died at the blood mobile unit. Why did they change it to a helicopter accident?
Answer: There was an episode where Linda was mugged at a free mobile clinic ("Under the Gun"), but I don't know of any episode where she was supposedly killed before the helicopter crash. Linda was alive until The End of season 7 and killed off when Amy Carlson decided not to return for season 8. Later, it was suggested Linda's helicopter accident wasn't really an accident and she was murdered. In "Common Enemies", Danny arrests the man responsible for her murder.
Question: What pier are Frank and Jamie fishing off? They've done several fishing scenes there I believe.
Answer: In this specific episode "Mercy" S2xE1, the final scene was filmed at Brooklyn's Valentino Pier (also known as Coffey Street Pier), in Red Hook. This area's full name is Louis Valentino Jr Park and Pier. The unusual structure and layout of Valentino Pier make it quite unique (and it's a lovely place to just hang out). Different local piers have been used to film fishing scenes in other episodes, note their distinctive structures and skyline views.
Question: What is the lapel pin Commissioner Frank Reagan wears on his suit lapel?
Answer: If I'm not mistaken, I believe you're referring to his NYC Police Commissioner pin.
Answer: The Seal of the City of New York.