Synthetic foods are made especially for humans. That sounds like a better start to me than something that has evolved by coincidence and which may try to protect itself with all kinds of poisons, that have e.g. been (partially) selected out by farmers in the past centuries or are simply cooked out as we do with e.g. potatoes.
When and if they fit your needs. If you're overweight and on a caloric surplus, eating peanut butter would very likely not be healthy at all. Again an overly simplistic view.
Says who? Not the WHO for one. Also not the EFSA, who control which substances can be added to foods in the EU and who would definitely not allow any dangerous substances to be added to food. They assess thousands of studies before they reach their scientific opinions and allow something on the European market. Here's an assessment of synthetic vitamin E.
But you've concluded that there wasn't enough research right? Read your own research and crafted your opinion. How many research articles did you read, couple of thousand?