... but didn't post himself? Should I mark it CW at least then since the rep shouldn't be mine?
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I'd first leave a comment on the original post suggesting that they should post the idea themselves. If they decline, feel free to post it yourself. I don't think you need to make it CW, since you're the one making the effort to post the suggestion. It won't do any harm to give a citation in the form of a link to the original post, just so people can see how the idea came about.
Yes. An idea is an idea, and if it's interesting, it is a good contribution to the site regardless of its origin.
I don't think it should be CW just because of that. Use it if you want to encourage collaborative editing.