I think that you are right, and that your daughter might have a past life issue. You have received some great advice above. All I can really add, is to just hang in there. (This is harder than it seems, I know, when your child is freaking out for no apparent reason.) I can tell you from a place of experience, it will get better.
If she were my child, to address fears of death, I would say something like: You do not have to worry about losing me as your mom for a long, long time. You have just started out. Enjoy being five, while you are still five. And then I would point out when her birthday is. That usually does it.
To discuss probing questions about the process of death, I have just been honest with my children, telling them that I am not really sure how they came to me, but that I am very glad they did. I also said, I do not think that there is anything in the world that you can do once, that you can not do a second time, so no matter what the future holds, we can always find each other. After all, they found me once before.
About the tasting food for her and possible poisoning, for now, I would humor her. Perhaps, get her involved making her own sandwiches, so she feels she has control over what she is eating. Let her watch when someone is preparing her food. Maybe talk about expiration dates and sealed jars so she is armored with some empowering knowledge. (But, then again, this could backfire. As an obviously caring mom, I am sure you will know when the time is right, if ever.)
Good luck! Let us know how you get on . . .