Weirdly, or not weirdly enough, Parallel Moon actually helped me come up with this theory in a way. I kept thinking how (aside from speculated/other factors) in that version, Usagi never became Queen, there's no Crystal Tokyo, everyone's just living more or less ordinary lives, right?
I kept wondering about it and remembered that during the Super arc (Infinity Arc) that the girls (Manga wise) were in their last year of Jr. High School/Middle school. Which, for them, is what the US would be 9th grade- aka, first year of high school for those of us in the US (and other places that have the same schooling system), making them all Fifteen.
All the Inners turned, more or less, Fifteen that year, which ends their compulsory schooling. After that year they legit don't have to do any further schooling if they don't want to. (Rei doesn't have to do any exams anyway, because her school is an elevator one- aka a lot like US schools in which she just gets promoted to the next grade level anyway.)
SO-
I kept pondering what the differences were in Parallel Moon and in the Canon continuity were. I had to first get rid of the hard-pressed feeling of keeping with the canon timeline and realize it's Parallel Moon- it's an AU that could take place a little earlier than what's normal. It also means a lot of the canon beats (Like Usagi getting married at Twenty-one to give birth to Chibiusa later and turn twenty-two.) might also not apply at all.
Like, for instance, Usagi and the Inners being born a decade or so earlier in 1967 instead of in 1978. (estimated/what most people agreed would've been their birth year with Mamoru's being in 1974-1975-ish).
There's no rule in-story of Parallel Moon that says this could be wrong. We only get one time stamp there and that's the beginning (when this short story came out) of July 1999. (Which is amusingly the month before Mamoru's birthday and a month after Usagi and Chibiusa's birthday.)
Chibiusa and Hotaru's given age would've been around 14-15yrs old (Mostly likely the latter) because it's stated that both girls are in their Third Year of Jr.High/Middle school. <- This here gives me an idea of when Chibiusa could've been possibly born, which, in turn gives me a rough idea of Usagi's current age in that one.
And I estimated about 17yrs old would've been when Usagi could've/would've given birth to Chibiusa.
This makes a lot of sense, if you factor in the aforementioned Compulsory Schooling and the legal age it is for people to get married without (or with) the consent of their parents. Currently it's 16 for girls and 18 for boys over there (I think. I could be utterly wrong. Laws change over time but at the time of the Manga's in-story years, this would've been correct, I believe.). Which is kinda roughly the same in some parts of the states depending, of course. (even though overall it's 18 here but I digress.)
Anyway-
My point being is that once I got rid of the notion that I absolute had to abhere to the main canon storyline time stamps, I found it a lot easier to estimate everyone's ages- and I used Naoko Takeuchi's birth year because she would've been that age at the time she wrote Parallel Moon.
I also had to go back and remember that during the SuperS/Dream Act (and it's been repeated before) what Usagi's actual Dream was that had nothing to do with her Sailor Moon/Princess things.
Her original dream was literally stated in the Dream Arct manga as being a Homemaker/Good wife and for her children to be happy and to have a nice home. (Because, her mom. She wanted to be like Ikuko and that's pretty cool.)
In Parallel Moon, she literally achieves this goal.
So, I figured that both schooling and her getting married earlier probably is what changed here between the Canon and the Parallel Moon world.
Plus, she still gets to meet the Outers anyway since Infinity arc happens while she was still in Jr. High. We can assume that Setsuna, like the other girls, was reborn similarly to Haruka and Michiru.
- Spoiler:
(Off. Off topic: The Time gate is the Space-Time Gate and she dwells in the Fourth Dimension which is completely different than normal. There's no telling who's at the gate or when this Setsuna reincarnated or if she knows anything at all. Afterall, it's Parallel Moon. Any number of reasons happened here.)
Long story short (lol) again:
Usagi choosing to continue her education and waiting until she was older than seventeen is most likely one of the reasons why Crystal Tokyo would've been way more likely to happen to her in the Canon Storyline. Whereas Parallel Moon, she settled down after schooling, maybe picked up a two-year thing and just lived life, thus Crystal Tokyo doesn't happen.
(other factors notwithstanding.)