Humans entered Westeros sometime in the past 12,000 years or so from the canon timeframe.
The Empire of the Dawn-seems to have been around the time of the Long Night and Asshai is of course...Asshai.
I would say human civilization has existed at a bronze or stone age level for 12,000-15,000 years. Perhaps a longer stone age in Essos, or perhaps Sothoryos.
The actual age of Planetos is also unclear. The Deep Ones and hints of an ancient fish like race-implies that civilizations have existed before humans.
It's a Ned on the Iron Throne fic, which kills my SoD on its own. If Robert and Rhaegar both died, it'd either be Stannis as king, or Viserys with Tywin as regent for his underage goodson.
To give a brief summary,
Ned becomes King and Tywin goes to the wall alongside Jaimie
Ned is betrayed and dies
Sansa is crowned due to Hoster's preference(Robb also is slain)
Sansa learns under the feet of LF, Tyrion, Hoster, Jon Arryn and her mother
Sansa fights a Targaryen restoration
Lots of people die
Sansa is captured
Sansa is tortured...a lot
Sansa escapes
Sansa wins the war
Sansa has become the ice queen
Sansa defeats her enemies and smiles at the wailing of their women
Sansa is now a great(as in bloody monstrous but also effective) Queen.
So as I said, its not a "Ned King" fic.