I read the Wow Comic with Mary Marvel's Christmas story and Mary Marvel's origin story in Captain Marvel, because I had never read it before, only having read a couple Captain Marvel and Superman and Batman stories in that genre in my entire lifetime (that determined that I wouldn't like "superhero stories"). The Christmas story was very expected, being the standard comic book Christmas story for children, that the local community's planned (charity) special holiday celebration and toy giving for underprivileged (usually orphan home-based) children is almost ruined by the toys or donated money for the party and toys purchase is stolen by the story's villains.
Because that story plot was so expected, this story was a bit boring to me. Had I been given the assignment of writing that story, which required that same scenario/basic theme and plot, I'd have been determined to "jazz it up" as much as possible. The best way to do that would be to use Will Eisner's tactic of making the villains interesting, by developing their characters much more than a normal "Cops vs. Robbers" plot, by using an unusual character trait or event in their pasts that makes their turn to a life of crime understandable, and makes them more "Human" in the eyes of the reader. Of course, that presages that the villains will most likely "be redeemed" at the end of the story, possibly voluntarily giving back the toys or toy money (after "seeing the light"), or by being caught, made to feel guilty, and then feel much better when they see the kids enjoying the party food and their received gifts.
As to Mary, and her newfound powers, I have the same problem that I have with Billy Batson becoming Captain Marvel, being pure magic, with no attempt even trying to use some subterfuge of pseudoscience to cloud over the obvious, at least to readers that don't think too deeply (as writers like Carl Barks did effectively). I have big trouble believing that simply enunciating the word, "Shazam", brings super strength and ability to fly (2 unrelated occurrences), and for Billy, it also matures him to a fully-grown man, while for Mary, it only provides her those 2 powers, but does NOT change her body physically, or her level of maturity. And IF just saying that "magic word" would provide those powers to anyone, how would that secret be kept from other Humans, including badly-behaved people. Actually, once the secret would get out, it would spread to virtually every non hermit Human, and Earth would become a planet of superstring, flying, practising "magicians" (superheroes and super villains).
That scenario leads to my problems with the original creator of Captain Marvel (C.C. Beck?) not having had explained exactly how The Gods decided to bestow these powers upon Billy Batson (why it was pre-ordained), and also upon his sister (later cousin?) Mary, to avoid the problem of any other Humans deciding totter "Shazam", and empower themselves. And what is the connection of that wise sage, who informed Billy of his "birthright", to the ancient gods? And is he an ancient immortal being, who lives in the sky (Valhalla?), or is he a present mortal, who lives in an old, abandoned subway train tunnel, beneath Central(New York) City? I would have liked those things explained in some way. Maybe there could have been a flashback to a meeting of The Gods, in which they have the gods and goddesses and heroes (half-mortal and half-human), who make up the "Shazam" anagram, donate their powers to the anagram's utterance in some visual magician's rite of magic (perhaps mixing a potion in a large cauldron?), with a ritual tossing of some powders representing those "powers", attributes, and skills. That event would be shown in a very dramatic splash panel, occurring in the black of night, illuminated by dramatic lighting from only the fire under the cauldron. Perhaps the ceremony could have had those gods speaking in Old Norse, Old Latin, or Old Greek (depending upon their particular cultural origins, and given asterisks, and whose text is translated to English in footnotes at the bottom of the page (which was done in Carl Barks' stories).
As to the idea that Billy Batson and Captain Marvel are two different beings, bringing up the question of whether or not they can exist in the same physical dimension at the same time (and react with each other as different beings) brings about a big additional problem. I would say it is a LOT safer to not go there. That later (DC) story in which that happens, opened up a big can of worms.
The bold action of NOT using some kind of pseudoscience to "cloud over" for the reader such problems of using pure magic, is what makes it impossible for me to enjoy "Superhero"-style comic book stories, because as a scientist, and a realist (even at the tender ages of 5 or 6 and above), would take me out of the story flow, to ask WHY? Whereas, in stories by Carl Barks, and H.G. Wells, and Jules Verne, and other classic writers, who used (1) either a general non-scientists', slightly educated instinctual guesses as to the reasons for things appearing a certain way, or (2) using pseudoscience to make non-understandable occurrences or known false assertions seem plausible, would allow me to stay within the story flow, and enjoy the reading simply for its entertainment value.