1. The problem with this is that depending on the type of reaction, and since it keeps going, we're going to not be able to 'freeze' the reaction to go measure. What we can do is to saturate the system at points to see the maximum output, and probably step it down from there slowly and use the data to derive the nature of the system (where slow-step happens, if there's a steady-state equilibrium, etc.) Of course, this would probably kill the subject. We could, of course, devise a scale model of a human body but I'm not sure how well that will work.
2. Metabolite-Metabolite interactions are meaningful, I believe, because if two metabolites are totally inert as individuals but become toxic/have some side-effect it may become a very good drug or a pair of drugs not to be administered together.
3. I think you can only get kinetic data from the former type.
4. I have no idea.