Some doubts that Niven ever was meant to be the focus --
Edwards nearly always does comedies, such as 10, Operation Petticoat.
The "caper" movie was an established genre by this time. The character of the "gentleman thief" also was a standard type, such as "the Saint." Niven had played one of the most famous, "A.J. Raffles," in a popular 1939 movie.
Having farce clash with genre was popular in this era. Abbott and Costello did about a dozen in the 1950s, such as Abbott and Costello Meet Frankenstein. Bob Hope did some similar ones.
In The Pink Panther the gimmick is that the focus is on the bumbling, Keystone-Kops-like police detective -- like Lestrade in the Holmes stories, also a stock figure -- with the suave and dapper gentleman-thief in this case in the background.
A famous "heist" movie came out about the same time, Topkapi. The genre continues, with Mamet's The Heist, remakes of Raffles, etc.
Sellers prepared for a year for the role; doesn't sound like anticipating a minor part.
And Sellers and Niven were together in the psychedelic 1967 version of Casino Royale (each, with several others, playing James Bond), with Sellers having a larger role. Doubtful that Niven would have been willing to do this if Sellers had taken the focus of 1963's Pink Panther from Niven.
So, IMHO, the movie from the outset was intended to be a showcase for Clouseau, with Niven playing a stereotypical gentleman-thief as his foil.
So I'd like to hear more evidence that the focus of The Pink Panther changed while it was being filmed. I'd be surprised an Oscar-winning super-star like Niven would have tolerated that sort of switch. But if you did a comedy with, say, Ray Romano in the lead and Sean Connery in any substantial role, it's probable that Connery, too, would get top billing.
The title of the movie of course refers to neither, but to the diamond itself.
Some interesting connections: Niven's unit in WWII was the "Phantom Reconnaissance Regiment," a kind of commando unit.
A few years later, Niven took the Clouseau role, as the police detective, in the heist movie Rough Cut, a little-known Don Siegel/Larry Gelbart picture with Burt Reynolds and Leslie-Ann Down as the jewel thieves.
Niven was a partner in Four-Star productions, which had hired Edwards as a writer in the 1950s.
Niven in Casino Royale was reunited with Deborah Kerr, his co-star from his 1958 Oscar-winning movie. Lots of such allusions in movies, meant to get a laugh from the audience.