Concretely speaking, you'd better to use "Daijobu?" with "desuka" to people older than you and those who you don't know well. You can use "Daijobu?" to your friends and children.

Yes, they are meaning the same. "Daijoubu desuka?" is more polite, and "Daijoubu?" is casual. You can use the latter one between close friends.

Yes they do. They are just different degrees of formality.