The Rachni wars went on an additional 2000 years without the Krogan?

No, they started much much later here. Alternate universe; some events are purposefully switched around.

Like Shepard being born a century later?

I can see how it might be the quickest way for Fallout humanity to join the Citadel races seeing as how none of them would try bootstrapping a post-nuclear apocalypse species if they had already done so to the Krogan. Unless they decided the Vaults and the NCR qualified as sufficient signs of civilization.

Tuchanka is untouched by the Citadel races in this timeline.

I figured, otherwise they wouldn't have bothered with FO earth.

Indeed, and rather than worry about the Genophage here, they have to worry about using Super Mutants in their armies as disposable shock troops, the mass killings of Ghouls, and the political mess caused by said despicable acts. It makes the other races suspicious of them (except the batarians, who are friendly to humanity here), the mutants hate them, and elements in their own numbers turn against them.

Then there's the turians to deal with. They're not a Citadel race here...

Why not? The turians were an interstellar power long before they made contact with the Citadel, and if they accepted the batarians I don't see why the turians couldn't get along with interstellar society, though probably not a Council member.

As I said, things are purposely different here in some places. Basically, in a lot of places it's a "polar shift" so to say.

History, not just humanity's either, went different here.

The batarians end up being slightly more mild, more willing to work with other races, and don't take slaves, the turians end up being isolationist and purposely avoid Citadel space until they have a sufficient fleet to conquer it, as per their colonialist views (unhindered by being involved with the Council here), and then they start a war against Citadel under Saren (who is here a General corrupted by Sovereign after an attempt to study and co-opt Reaper tech went awry).

There's a rebel faction of turians that are pretty much the exact mentality of the main-verse turians, of which FO:ME's Garrus is a part of.

Time is a fragile thing; one change can cause one hell of a difference later on.

This, by the way, was the best synopsis I've ever read; astonished at the exactiness