WW with no Greek mythology cannot work, no matter the tone, no matter the approach.
They're not on Nolan mode, you cannot have a flying humanoid alien in Nolan mode.
There's no way to do the lasso of truth without magic without being stupid.
Everything Kryptonian isn't any less magical than abracadabra stuff, it's just not.
Why do they need to explain that? Magic just hurts Superman, what is there to explain? Why does krytonite hurt Superman? Or a red sun?
Also, it doesn't matter if Shazam is "not part of the universe", it still establishes that magic exists in that same universe.
Again, there is
nothing, not a thing, "realistic" about Superman, nothing, and there is nothing realistic about a rich guy who can build an armor to fight Superman, also, an island full of women descended from myth isn't difficult to explain:
1) They are not all super powered, only Wonder Woman, the rest of the amazons are just good fighters.
2) There are only women in the island because all the male babies are discarded and sent to work for Hephaestus.
Yes, what's wrong with that? "False god" is a response from the people to Superman, not a factual declaration that gods don't exist, I don't see a problem there.
Besides, Wonder Woman has always been a part of that gods controversy in the comics and Lex Luthor fear mongering towards "gods".
And that was a dumb thing to do by the MCU.
That has nothing to do with Wonder Woman, and it doesn't limit Wonder Woman either.
Lke I said, they are not making the heroes realistic, just the context, the world and how it reacts to a silly ridiculous concept like an alien, that shots lases from his eyes and flies, and a demigod rooted in greek mythology isn't any more ridiculous and silly than that, you can't have Superman and be alright with it, and think that Wonder Woman is too silly to work, they're exactly the same.