Maybe I'm missing something but how did Don get to where he is now? Obviously he is v.smart and charismatic, but one has to have the right opportunities to get to Sterling Cooper.

So what's Don's time line? Army, then college while working as a car salesman, then ...? (I vaguely recall Betty mentioning that he was some sort of saleman/ corporate guy when they met while she was on the fur coat modeling shoot.)

Could someone fill me in with bits and pieces revealed over the episodes, plus your own theories to fill in the gaps? Thanks in advance.

I don't recall that "Don" went to college. The real Draper did, so maybe he has a copy of the diploma, or got the original from Anne, along with other Draper ID.
As to how he got into SC, your guess is as good as mine. I don't think they've ever explored it, but I've been curious as well.