OK, I’m responding to the page!
First of all, I will happily pimp my Legion help file which I’m amazed to see is still hosted on the linked site. The potential problem is that it is in the format of the old “Windows Help” which might not have a viewer application anymore, but given the breadth of the internet, I have no doubt a usable one can be found.
It is quite old, I have not updated it since about 2004. A large part of that was because shortly after that point, DC started messing with Superman’s history (Mark Waid’s “Superman: Birthright”) with zero regard for how it affects the continuity of the post-Zero Hour world. I had a lot of bytes invested in getting post-ZH Superman, Superboy, Brainiac and Lex Luthor clear within the context of Legion history, and now, it was pretty much all gone.
But to get to your questions: The Legion can effectively be divided into four sub-histories:
Pre-Zero Hour, which encompasses two reboots - but those were in-continuty reboots
Post-Zero Hour
Waid and Kitson’s “Threeboot”
Geoff Johns’ “retro-boot” of the first.
Before the Legion had a regular feature, consistency was a pipe dream at best. The writers said in the story whatever felt good at the time. But from Adventure Comics # 300 all the way until the Crisis on Infinite Earths, the writers paid attention to the history, and even attempted to reconcile some of the mistakes of the ultra-early writers who didn’t care.
Then, the Crisis happened, and the reboot of a Superman as a hero who was never Superboy was mandated from higher-up, as was the non-existence of Supergirl. The Legion writers addressed the Superboy problem by having the Time Trapper create an alternate dimension with a Superboy who matched the Legion’s history. Supergirl’s absence was ignored, which was not really problematic, she was instrumental to very few Legion stories. Things were fine for about three more years.
Then, in 1990, the idiot in charge of the Superman books - who wielded the most power in the DC comics offices - decided, for no reason that I could ever fathom, that the Time Trapper’s Superboy had to have never existed. Beholden to this mandate, the writers of the Legion book put together an in-continuity timeline change, the result of which was the Legionnaire formerly known as Mon-El was now to be called Valor and was the Superboy-equivalent in Legion history. (A Supergirl-equivalent was also introduced, but that doesn’t really contribute to the confusion.) Then, a few months later, that idiot woke up and realized that removing that story from Superman’s history would have reprecussions for his books (e.g., the introduction of a new Supergirl in the Superman books, and her attraction for Lex Luthor, were important plot points that could not be explained without that Superboy story, so he told them, “never mind, put it back in”. The Legion writers sighed, cursed under their breath, and wrote a little backup story which put the Time Trapper and his Superboy back into a never-well-explained (though in my help file, I come up with a pretty good retcon for this) limited slice of Legion history.
Finally, four years later, DC felt a need to present readers with a simple introduction to every character they were publishing, to be available in a single month’s issues, so that new readers could on-board easily rather than need to hunt down back issues. Mark Waid was the man chosen to do this for the Legion, and he decided that it simply couldn’t be done. So he made Zero Hour a nice capstone to the Legion history that came before and re-started it.
This began the second era of Legion history, the “Archie Legion”, which for the first time ever presented a Legion in the process of formation, where new readers could grow with the team, and nods to the prior versions would be appreciated by old readers. This was fine for a number of years, but eventually, readers grew less interested in the stories (which were, in fact, less interesting) and the numbers dropped. They brought in Dan Abnett and Andy Lanning to try and revitalize the property using the existing team, but that boost proved to be temporary, and the Legion was headed for cancellation. It was also during this time that DC pretty much gave up on caring about historical consistency. Changing Superman’s origin and re-introducing the dead original Doom Patrol are among the cases where they just went ahead with what they felt were good stories without any thoughts to the implications on stories that were previously published.
So DC turned once again to Mark Waid to revitalize the property, and he created the “threeboot” in which the Legion is sort of a youth rebellion in a restrictive, controlling environment, another case where DC went with a story that ignores what went before. While the team were new versions of familiar characters, the villains were completely new, and the team was introduced in situ, their origins never, to this date, explained. Readers failed to connect with the team, and after three years, the book was cancelled.
Finally, after a few years of no Legion presence (except for the Starman of the JSA being a time-displaced Star Boy), the original Legion - based on a modified version of the team as published in 1989 (but somehow with Karate Kid alive - did I mention that DC stopped caring about continuity?) was re-introduced in the pages of Justice League of America and later, Superman. These appearances led into the “Final Crisis” event, which included a mini-series called “Legion of Three Worlds” in which the three versions of the Legion - the modified-1989 version (anything published between 1989 and 1994 was completely ignored, except for a brief cameo appearance in a double-page spread) are said to be from alternate Earths (a concept re-introduced after being eliminated in the first Crisis) and they fight pretty much every Legion villain there ever was. The conclusion was to establish a Legion roster going forward, which was the modified-1989 version plus two characters from the post-Zero-Hour Legion. The other two Legions were shuffled off, one to explore the multiverse and one back to their home dimension. Mon-El’s/Valor’s new 20th-21st century history was established in a Superman storyline.
That’s the official history to this point, not counting whatever DC has introduced in its “Rebirth” event, which I have very little knowledge of. The roster that exited the Final Crisis event was pretty much untouched by the “New 52” reboot (though a second book had seven Legionnaires time-displaced in the 21st century) and when the book was cancelled for low sales, they dropped a hint that the Legion was from the future of new-52 Earth-2 rather than from the future of the main Earth on which most of the stories DC were publishing took place. But that was left a little ambiguous, and they appeared in a Justice League storyline which seems to have the Legion and the Justice League on the same Earth.
Whew! So, did that answer everything?