What was the ancient land called, before being named atulos?
The ancient land was called Arkaria, named after Arkahron himself, the ancestor of Atulos.
What sort of year are we on in game? eg 1000AA (After Atulos).
The year, or number of years is unknown, other than it has been many generations since Atulos ruled.
That said with the time since atulos's reign do the eras have any relevance to the time line at all?
After Atulos was originally killed, there was a period of initial peace that eventually gave way to war between the guilds as they tried to claim their share of a land with no ruler. An era is a non-specific but significant period of time that is defined by the war heroes that dominated each era. Some are
recorded and known, and others are forgotten.
Are we going to see these mountain rebels or have they been disbanded since the downfall of atulos?
The mountain area was originally created as a direct reference to the mountain rebels, although the rebels have long since disbanded the mountain still remains.
And finally is there any back story to atulos that hasn't been made available yet just sitting around? 
Yes