But the original conception is that not only does Athas share the Prime with other settings (and genres, etc.), but Athas shares the Prime with other Athases!
That isn’t Gygax, that is 1960’s Star Trek.
One week the Enterprise using ‘normal’ propulsion, travels to an Earth analogue that suffered a global thermonuclear war, that the Enterprise’s Earth did not suffer through.
Next week, a rarified accident sends Kirk to the Mirror Universe where Spock has a beard.
A set of infinite possibilities just means one can do
anything, even contradict oneself from session to session. Infinity
includes violation of the law of the excluded middle and the law of non contradiction.
1. First, characters are explicitly portable between campaigns. This is something that always used to be assumed, but (for whatever reason) rarely happens anymore; part of the reason is because the conception people have of the setting has changed.
The reason for this is less cosmological and more a change in play style. Most games today, start with world building, and then plot, and then the dungeon.
Early D&D was the opposite. You had the dungeon. Two weeks later, another Dungeon.
World building was optional.
More importantly, an advanced character coming in from another, unknown DM’s campaign often lead to upset people, in my experience.
Either the DM did not vette the incoming character, and the power levels where quite disparate.
Alternatively, the DM did vette the new character, and the player was upset at being nerfed.
The Lake Geneva crew was a fairly calibrated group of DMs.
Once Gary Gygax started playing with PCs from other campaigns, Gary became angry and vengeful,
and thus created Tomb of Horrors, to rid the multiverse of Punk Ass Player Characters.
The genius, the OP applauds is
evil genius, spawning cosmic horror 

