DT, a GM has to be an enternal optimist.
Right now, I don't quite have the time to give as lengthy and polished reply as I would like but I want to y'all's questions sooner rather than later.
You may make your character's background as long as it won't work against the game (and trust me, I am hard pressed to think of a character that wouldn't work) OR I will come up with a background for your character, whatever YOU want. You can even be one of the characters in the Amber series (we will assume there is a very good reason why such a powerful being has been reduced to an ordinary teen).
Don't forget, there are infinite variations of the Amber family themselves throughout Shadow. Even Corwin can't be sure if he is fighting for the One True World, or just the One True World he knows of. So we don't have to stick to established continuity. And characters can be effectively duplicated, slightly different versions from slightly different shadows (different brands of Brand, for example).
Also, if every possibility happens in Shadow, then every story or character you have read about has happened somewhere in Shadow, you may be able to bring your favorite character from ANY story into the game.
In short, just suggest a background, 99% will work. The only real caveat is that your character is somewhat more than a self-centered would-be tyrant (this includes the Princes of Amber, but your prince may have had a moral awakening. Besides, given Eternity, even selfishness or mere power would get old after a long while).
At game start, most of the people around your character (including your character) will think the PC to be somewhere around the ages of 11-17, the characters true age (chronological, apparent and mental) is up to the player.
Given that different shadows have different timeflows and that these timeflows can be set (relative to Amber) by powerful enough beings, I can easily set the game to whatever period of the books the players want.
More specifics about the game will depend on the particular mix of player preferences. For example, if every player wants to be the descendant of a particular Prince (do Amber Princesses have children?), that will be a different game than if the players want to be an Amber breeding experiment from the Courts of Chaos.
Here are (as I see them) the two basic approaches to the game, with near-infinite variations between the two...
About a generation (subjective time) before the game starts...
A war for the Pattern is fought between two powerful forces (Law and Chaos?). One side wins, but not decisively. The losing side is able to save some of the children and warriors of their side to safety. As the exiles are being tracked not just physically, but mentally, spiritually and magically, it is necessary to disguise the rebels as children, physically and mentally, even to themselves. A group of powerful guardians is caring for them but their pursuers are more powerful and discover them early...
APPROACH ONE: The rebels are being hidden by what is left of their family retainers. There is no question of their loyalty to the kid's initial cause.
APPROACH TWO: The kidnapped kids are being raised by their enemies who hope to raise them as powerful warriors for their side and eventual comeback.
Whatever the true motives of the kid's guardians, it is necessary to them that the kids be raised in a happy environment with a need to right 'moral wrongs.' So the game will start with what seems a truly loving family. This will add drama later when the character's realize their true selves and learn they are the children of mortal enemies, or the mortal enemies themselves.
I mean, Corwin and Eric couldn't have hated each other from their cribs. And their mutual hatred becomes more poignant if they once loved each other.
As far as bidding, here is my provisional idea...
All characters start as HUMAN in all stats, with no powers bought or points bid. As the game progresses, the character may bid to their full 100 points, freely at times, restricted at others. This is the game effect of the children slowly realizing themselves to be demigods. Your character's true self may have been a 500 point uberhero but this game deals with the short span of their eternal life where they remember who they are or once were...
So, let's say that Brother A and Sister B have learned that they are mortal enemies and are fighting for power. Both can bid for their Warfare stats, but once those points are spent, they are committed to that stat. So, both siblings have to decide how important is this fight and the Warfare stat to the rest of the game. So the player with the highest Strength in the early phases of the game may not be the highest at the end of the game if another player really wants it more.
I'll stop here for now, let me know what you think. Nothing is cast in stone and I'm not married to anything...