Its your game and all, but since i kinda hijacked the setting by writing more than everyone else, meh.
I disagree with your ideas. Firstly, you say everyone abandons earth after its resources go...
a) There are _currently_ 7 billion on earth. Do you realise how _long_ it would take to move them? Not everyone would want to.
b) Why would we abandon earth? It's where all our religious, historical stuff is. The muslims would be against abandoning mecca for example. If all our infrastructure is here, all our population and all our institutions, then we would simply ship in supplies. Not run away. After all, london is a long way away from stuff like coal pits and what not, yet it has 8 million people out of 60 million brits. And in a space scenario, not much is different.
So i am voting for anything that involves earth remaining as centre of the map.
Plus, figureing out how the factions fit together is vital, in order to work out how they would have evolved, how they would equip their armies and so on. Nothing exists in isolation, and so we must, from the start, work out how the pieces fit together.
And its a damned tabletop game! How can the factions _not_ know about each other? They will face each other on the tabletop, and you can hardly make a table saying who can fight what.