Well, presumably you’ve had injections or medical treatment which rely on your body being consistent with humans that have been studied; you could claim that as evidence that your body reacts as would one of carbon-bearing oxygen molecules. But if there was no such link to your particular case, i’d say no, there’d be no evidence.
Sort of. The strongest, informal phrasing which would probably be most common is “P is true”. A scientist would tend to use your latter example when talking about a particular case (and what I would consider the most accurate phrasing), and the former when talking about a theory.