Final Fantasy Crystal Chronicles: Remastered will be released on August 27, but the original GameCube version will not feature any major features. The game’s Japanese Twitter account has confirmed that the game will only be played in online multiplayer and you cannot set the session for an offline event.
The tweet below, roughly translated, states that the game lacks offline multiplayer and prioritizes online play so that more people can play together.
With Crossplay enabled, there’s nothing theoretically stopping Switch and mobile players from playing on separate devices, at least in the same room. However, if you want to bring four actors together on a single TV, you don’t seem to have much luck.