About the man in the background in the third panel:
He is sufficiently generic that he could be one of several characters (Archie Goodwin as mentioned, Sam Spade, Philip Marlowe, Steve Carella, etc.), but I prefer to think that he is Mike Hammer, included as a tribute to Max Allan Collins (long-time writer of “Dick Tracy,” and now author of the Hammer novels).
No, Chang Apana was mentioned as a real person in the storyline that introduced Doubleup.