It would need to explain where the dragonborn came from
Okay. That is true. I don't see that as a problem. There are many ways to handle that.
and why there are so many tiefling
Points to Horned Society and the Evil Empire of Iuz the Cambion I don't think that one is much of a stretch. Evil devil-worshiping countries can be full of people who have the touch of the demonic on them. The worse that will happen is people will say "but 40 years ago that wasn't true!!!" and... so what? That doesn't mean it isn't completely in keeping with the setting. And remember, your point was that A) It doesn't work and B) New PLayers who don't know the 40 years ago lore. SO the cries of "but what about 40 years ago when that wasn't true" doesn't apply to either of your actual arguments.
and why all the orcs aren't evil
Good aligned Orcs existed in Greyhawk 40 years ago. Heck, the entry in the book even said something along the lines of "and humans suck too". This isn't even a change.
and why the psionics is different
Needs zero explanation. Psionics are mechanics. We don't need to explain mechanical differences.
and where the crashed space ships came from
Weren't those already in the setting?
and why evrrrrbody and dey mama has Cantrips and spells and why feats let everyone tap into there classes so easily
Did the setting ever specifically state that feats could not grant magic? Or that casters could not cast small spells with little issue? I've never read any setting that specifically called out these mechanics. Heck, I don't remember reading an explanation for why cantrips now exist in Eberron when they didn't before.
And again, the only people who need this explained are old fans, who don't like there not being an explanation beyond "the last time we wrote for this was two editions ago". And if you truly, deeply need this explanation... Vecna. Vecna just tried to rewrite the multiverse and was banished back to Oerth. That's why magic suddenly changed. Tie in the with the already published adventure that already goes to Greyhawk as part of the adventure.
This is a nothing burger.
and why the game is "Big Damn Heroes storm Iuz's castle" over "Plucky Jerks influencing Local Politics"
Was... Was fighting Iuz never something the party was supposed to do? What happened to the entire Vecna series where you start in Oerth and go to sigil, as Vecna ripped some of the divinity from Iuz?
Like, I can't believe that Greyhawk NEVER had an adventure focused on taking down a terrible evil, considering it featured Acerak, The Elder Elemental Eye, Multiple Demon Lords, Vecna, Iuz, and others. This seems like something you made up as a fact about Greyhawk.