"かい？" always means as same as "か?" or "ですか?" in a canversational casual speaking way. So you could use it to your friends; however, It sounds a little bit "a condescending expression" for me. For instance, your boss would use this expression to you. But like I mentioned, this expression also could appear in a conversation between friends. (Though I personaly don`t use it so much, and I think this expression is not really a common expression.)

Yes, it is commonly used and the same as か. In my opinion adult male speakers prefer to use かい to か when they wish to give slightly more frank impression. e.g. そうかい？(Is that so?) ぼくの財布(saifu)知らないかい？(Do you know where my wallet is?)

youg

"～かい？" Young people are not using much. :D 若い人はほとんど使わないよ（＾о＾）