I find the idea of Superman first arriving at the climax of the film and gaining every hero's support without question, despite not having met most of them, to be just as much of a head scratch.lol
My guess is Clark will appear in the second act as a plot device to unite the team, and by the end he'll be the leader.I've said this before, but I think by the end of JL1 we'll the confident Superman that everyone's been asking for. Personally I'm enjoying watching Superman grow throughout the films.
Superman will be the one to get Aurthor to join the team. Superman's death and return will inspire Aquaman to finally joint the team.