Joshua: I'm a little bit confused, so I'm hoping you can clear this up for me. If, in your hypothetical situations, you weren't planning to market the first two "situations" and only the third, where would you have credited these people in the first place, and why? Crediting is only an issue with tricks that you market anyway. I'm not trying to be rude, I'm just trying to understand what you're saying, thanks.
I was thinking of the only place where you give credit outside of marketing: while performing for magicians.
Where I do disagree with you, though, is in scenario two. You say that because you watch someone else's performance and create "your own" method (which happens to be the same), it makes the trick (or the method, I'm not sure what you're referring to--either way, it's beyond the point) just as much yours as his? Aren't you ignoring the fact that you watched
his performance, and created a handling based on
how you saw him perform it? I wouldn't consider someone who does that an "inventor" by any means...neither the idea nor the handling would be yours. The reason I say the handling wouldn't be yours, despite your coming up with it "independently" is because when you watch a performance, as a magician, you get certain cues as to how something is done--it's hard not to tell when someone uses a double lift, for example, when you see it--there are just certain cues that you pick up on as a magician. Whether you do it consciously or not, you are picking up on it, and it's influencing your "creative process" from then on.
I dunno, that's just how I see it.