J.C.
Wow, I am glad I'm not alone either. My mother always called me her disaster child. I had an obsession with "what if's". My son is 8 and is asking those sorts of questions too. What if you die, where will I go. What if the car falls off the bridge. What if I lose you in the store, how do I find you, etc.
I talk to him about those worst case situations so he has an idea of what to do. First, I know that some of this is normal. It becomes a problem when it inhibits you from living life normally. Second, I agree with the others who say that you should probably seek help either from her pediatrician or counselling of some sort.
I wonder if you took her aside one evening and let her ask you all the questions she could think of so you two could work out a plan or ease a worry with an explanation about how things work would help.
I do that with my son. I lay with him in his bed and let him ask away and we talk about it and he doesn't ask about it anymore because he knows what to expect.
Oh, one more thing, my son has A.D.D. and has been on medication for it for the last 2 months. The questions are becoming fewer and fewer.
You may even consider getting her a diary so she can have a place to put her thoughts so she doesn't have to dwell on them.
Ok, sorry for the brainstorming. I wish you well and loads of luck.