What you’re referring to are the explanations given for both post-Crisis Billy Batson and Mary Marvel possessing an adult form when they transform.
I can’t recall the specific issue off of the top of my head, but I recall in Power of Shazam! that the reason Mary had an adult form was because she most admired her mother the same way Billy did with his dad.
In terms of specifics, that’s the only example exactly like that.
There are similar precedents, however. For example, during the Ordway Power of Shazam! era, Mary Marvel didn’t go by “Mary Marvel,” but by “Captain Marvel.”
As for seeing this naming convention utilized elsewhere, I don’t see the necessity for it. “Mary Marvel” and “Captain Marvel Jr.” works just fine.
Physically, outside of his Captain Marvel form, he’s quite vulnerable as Billy Batson. As Captain Marvel, there was a time where he was as vulnerable to magical attacks as anyone else.
In terms of intangibles, he’s been quick to lose his temper or get angry, so it’s something that can be used against him.
I’m not sure what you are asking, but Billy is always aging when he is in his mortal form, no matter the version. In Kingdom Come, which was basically an alternate version of the character, we saw a Billy who had grown to adulthood.
Thunderworld Adventures truly is a spectacular story, but I’m not sure I would recommend it for new readers. Similarly to All Star Superman, a new reader to Billy can enjoy it on it’s own merits, but it has flavors and nuances you will pick up when you re-read it after learning more and reading more Captain Marvel. All the Convergence stories rely on some pre-knowledge but that particular two-parter features some spectacular art, including one of my personal favorites:
Even though I am not a fan of Jeff Smith being confused about the powers and role of the US Attorney General, his Monster Society of Evil mini-series is one I would readily recommend to new readers.The Magic of Shazam, since it’s aimed for the younger set, is also a good choice.
Like its name would indicate, it’s simply a place that’s the opposite of the Rock of Eternity.
It’s apparently a place where its own mirror opposite of the wizard Shazam resides and seemingly seeks his own champion(s) of evil, as the statues of the Seven Sacred Virtues Of Man would indicate.
The whole saying it with the intention to transform began with Curse of Shazam!
There, the wizard Shazam (before being revealed to be the aborigine god, Mamaragan) states plainly that it has to be said with purpose, belief, and good intentions.
This was likely Geoff Johns’ way of answering the question of, “How does he introduce himself if he can’t say his name?”
Since then, for that version only, as you correctly state, it’s gone back and forth. Well… more accurately – and ironically – it’s gone predominantly, if not exclusively back and forth in Johns’ Shazam! series.
In Shazam! #1, where Billy and his foster siblings are trying to come up with codenames, Billy outright says that he wants a name that he can say.