Yes, these are all common questions. Brandon had intended to write the sequel, Nightblood, sometime soon after Warbreaker's release which would clarify these things, but he's answered the first two, with the last probably being secrets for the sequel.
1 - Vasher liked to hide his divine breath, thus concealing his true identity. This involves what Brandon calls "fancy mental gymnastics", but basically, it all comes from how the Returned see themselves. Those who see themselves as strong will get stronger. Here, Vasher convinced himself that he was not a god, and therefore the divine breath (the one that grants the Fifth Heightening) was for all intents and purposes, hidden. So with this technique, he could switch from Fifth Heightening to something not of that, as he was in most of this book.
2 - Another very common misconception. The royal line was descended from the First Returned, not Peacegiver. The other Returned were separate from this. Vasher and Vivenna are not related.
3 - You are correct.
4 - We don't know the precise mechanism for how things are Returned. Both of these will probably be in the sequel.