Theory: Marion Club's Location

Abstract
The Marion Club and the other nearby golf courses from Mario Golf (GBC) and Mario Golf: Advance Tour are located on the Californian coast.

Arguments

 * Argument 1: Marion was most likely founded by Mario, as shown through its name and his face being seen all over the club, and thus it does not have to be the same location as the real life Merion Golf Club in Pennsylvania. Mario could have just been inspired by Merion’s name.


 * Argument 2: The other courses in the games are within walking distance of Marion, meaning Marion must be very close to the ocean.


 * Argument 3: The Dunes course looks just like the deserts in the Southwestern US, and with the course being so close to the ocean, it must be in California rather than say, Arizona. The palm trees in the Palms course also suggests a more tropical location, such as Southern California.


 * Argument 4: Mario and Luigi debut in their golf outfits from NES Open Tournament Golf in Mario Kart Tour during the Los Angeles Tour (2022). The Bros must associate California heavily with golf - maybe because it is where Marion is located.


 * Additional Info: While there are many golf courses in Southern California, perhaps Marion is more north in the state: specifically, at the Pebble Beach Golf Club, one of the most famous courses in the world. The coastline matches up quite a bit to the map seen in game, and there is a nearby sandy area that could match up with the Dunes course.