To be honest, there is virtually nothing more to the story than "Farsight's Ethereal died, he carried on fighting the Orks and never came back".
At some point it was decided by the Aun'va that he had founded his own fortified colonies out in the Damocles Gulf, but since he had cut off communication with the Empire before that happened it's not explained how anyone knows this.
As far as the Players are concerned, any of the above theories could be true or none of them.
We don't know who Farsight has been in contact with - or even if, historically, he's even alive. Even that the Dawnblade is an Eldar weapon is just conjecture; all the Codex says is that it was found 'on a dead world', in the 'pre-human' ruins of an 'unknown' enemy (which suggests Necron as much as it does Eldar).
I'll take your word for that one - last I knew, Iyanden was right on Tau's Galactic doorstep, and Alaitoc wasn't incredibly far away from that.and according to the map they lies in opposite sides of the galaxy.