Why the HECK would Diana and Arthur mince Superman up into tiny pieces? They're heroes, remember? They don't kill fellow heroes just because he lashed out at them in confusion, or whatever the circumstances behind this fight happen to be. It's called "holding back." And if the other guy isn't holding back, that tends to go badly for you.
Shazam? We'll have to see how he does. Historically, he's been on Superman's level.
And Superman has NEVER been portrayed as the most skillful member of the League, so I don't know where you're getting that.
Context matters here. If Superman beats the entire League, it's almost certainly because he was playing for keeps and they weren't. If he beats Steppenwolf with absolutely zero assistance from the rest of the League? That'll be a problem, but there's no evidence yet that that is the case.
EVERY comic character gets "nerfed" for the purpose of story. Superman is like the KING of plot-related nerfing, in fact. If Diana has one moment of not looking like the most awesome character in the movie, that's hardly an insult to her. It's just the way comics stories go.