Yeah yeah it's not 100% but it's still reasonably strong evidence. If you find something in a plant that's eaten by many things then it's probably not very toxic.
Maybe that's some evidence, but it's certainly not reasonably strong (in fact, it's very weak evidence). First, some of those "many things" would need to have a metabolism that's close to ours. Secondly, even if those unaffected organisms are similar to us, we would need to consume that molecule at similar concentrations. Most toxins have adverse effect only beyond a certain dosage. Something can be harmless (like cyanide) in it's "naturally occurring" concentrations, and positively lethal in higher doses. Besides, synthetic molecules are tested before they're approved for human consumption, and they are usually tested on animals with similar metabolism to us, and at the appropriate dose. So actually, FDA approval is a much, much, much stronger evidence for a drug's safety than it being extracted from "nature".
