It seems to have a direct analogue in German as "denn" - e.g. "Wer denn?" where it is used to make the question sound less like a command/demand. Just asking "Wer?", "Wo?" etc. sounds somewhat harsh and impolite. Does då have a similar function in Swedish, or is it a grammatical "false friend"?
There's another use of då, at the end of questions longer than one word, that is often used to express impatience. So då in Vem då? is neutral and carries no meaning, but if you add a second då, you get a more impatient question: Vem då då? which might be translated as 'Who then?'
Also see what I just wrote in response to hoyunmyoun, that's my current theory.
Neither really. I think it's a lot to do with intonation. Two syllables are more comfortable than one for us. For instance we like to "shorten" one-syllable names into two-syllable nicknames. So people who are called Per, Stig, or Lars can be called Pelle, Stickan, and Lasse informally – and somehow that feels easier to say.