Or, in a nutshell, the multiverse became confusing because the people who said it was confusing made it so.

As James notes, above, appearances by multiverse characters were handled quickly and easily, but, alas, we were starting to drift into that prolonged period where those hardcore (read "anal") fans complained about "being told what we already know." As the marketplace shrank, there was less and less concern for new readers -- and, as a result, fewer and fewer new readers.

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That's very much a "be careful what you wish for" scenario. The engineers behind CRISIS were accepting things as they were -- as they were in their own heads. Which, in their arrogance, they deemed too confusing for "new readers".

feh

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Earth 1 and Earth 2 -- which really should have been Earth 2 and Earth 1 -- as originally presented were basically the same, except that their timelines were different. The beginning of the Superhero Age had been earlier on Earth 2, but basically all the same characters had appeared, albeit in different forms. Think of it as the "Mirror, Mirror" episode of STAR TREK, except that in this case there was no good/evil division.

Based on this, explaining the Earth 1 version of ANY character should have been easy: they had the same history as their Earth 2 counterparts, just occurring later.*

A big problem for DC in setting up CRISIS, of course, was that some Golden Age characters had unbroken runs since their first appearances in the Thirties and Forties. Superman, Batman, Wonder Woman, and even Green Arrow and Aquaman, had not "gone away" like the Golden Age versions of the Flash, Green Lantern, the Atom, Hawkman, etc. So those anal fanboys started asking where the "break" occurred. Which adventures belonged to the Golden Age Superman, and which to the Silver Age. My response, all along, was to assume (and later to say) they'd both had the same adventures, just at different times. Ditto for the rest.

But asking DC to take the easy way is pretty much the definition of wasting one's time.

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* And "later" is one of those things readers were simply going to have to accept as a sliding scale. When "The Flash of Two Worlds" happened, "later" mean nineteen or twenty years. By the time CRISIS happened, it was more like fifty years. But those were the same length of time!