A natural person. That's what CSAM covers. There have been prosecutions under federal CSAM laws otherwise, but there have also been successful constitutional challenges that, briefly, classify fabricated content as obscenity. The implication there is that private possession of obscene materials is lawful.
Agreed. That seems to be the way that kind of thing would play out. One observation: I've read opinions (one or two) that have qualified "depiction" using a term like "indistinguishable" or similar. (I'm having trouble finding one on the fly, but I think it was out of Wisconsin.) I think they're looking for a way to characterize it regardless of the medium and with the intent to lead an observer to believe that the depiction is of the actual person. I don't think it's enough to create a caricature or cartoon bearing the likeness of a real person, e.g., the trope of portraying a political figure as a naked baby.