Answers to your questions....
1) Yes and No. Stories can be in any POV, but you have to think of whether it makes sense to your story or not.
2) No. That can be considered fanfiction, or just used as an inspiration. If you decide to publish it, especially if you are using the same characters, places, etc. , put a disclaimer before you publish it.
3) Either way will work. Again, you can use the characters or make up your own, just, if you do decide to publish it, you have to use a disclaimer.
(Hope that helps)