Would Princess Serenity have still become Sailor Moon?
No. No reincarnation on Earth means no Sailor Moon. She would still be the Princess the Sailor Soldiers protect. There would be no reason for that to change.
Would she and Prince Endymion still have been together (perhaps abdicating their thrones to do so), or would they have ended their relationship to appease their respective kingdoms?
Yes, they'd still be together. It's not called "Miracle Romance" for nothing, you know.
And yes, they would still have remained in their respective positions of power. It's the only way of life they've ever known. Not to mention, I think they love being royals since even as humans on Earth they wasted no time establishing their reign again.
Would the Four Generals and the Soldiers of the Four Guardian Deities have ended up together?
Maybe. They would probably have to spend a lot of time together to confer on how to keep protecting the royal couple and the kingdoms. On the other hand, love doesn't work so neatly and conveniently like that. Perhaps a love triangle would have developed or something even messier. Who knows?
Would Saturn have ever awakened?
That's difficult to tell. It depends on whether a new threat ever appeared.
Would Uranus, Neptune, and Pluto have always remained in isolation?
Probably. They really did get the short end on the stick during Silver Millennium, didn't they?
Would Artemis and Luna have still become a couple and had Diana?
If no other Mau inhabitants migrated to the Moon and they were the only two of their species there, then yes.
Would Chibiusa (assuming Serenity and Endymion did stay together and had a daughter) have still suffered her weird stunted aging thing?
Yes. Even though I hate that stunted growth nonsense, it's safe to assume that whatever mental issues Chibiusa had developed because of her position as the princess, would be more or less the same since her role and the pressure it carried wouldn't have changed.
Would the Black Moon Clan have still arisen?
That's entirely plausible. If history has taught us anything, it is that there will always be people who are against monarchy as a system of rule and are willing to fight against it.
Would the Death Busters have still invaded Earth, or would the Outers have detected and defeated them in time?
I say yes. Even if the Outers detected them, they wouldn't be able to defeat them on their own without the power of the Silver Crystal.