I've only seen a few attorneys around this forum [and I'm not one of them either]. So theory discussions are much more practical...
Your post brings up some great remarks and while I've had some solid courses on copyright law, I'm not sure how where changing the elements impacts the rules if the outcome is the same and you knowingly base it off of another work [derivative works]. For example, you can hear a song, hire someone to record it, and still get sued [and lose] for infringing the copyrights of the words. Similarly if you see a painting that's still under copyright, you can't paint your own and sell it even with your own brushes, canvas and paint. It's still a derivative.... But would it violate anything to change the words and film the exact same story for a music video?
That's the best parallel I can think of for a website: if you built the same thing yourself without knowing the other existed it would clearly not violate their rights but if you clone it I'm not sure having unique code would get you off the hook.
A couple remarks from the copyright book via Wikipedia
A “derivative work” is a work based upon one or more pre-existing works, such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which a work may be recast, transformed, or adapted. A work consisting of editorial revisions, annotations, elaborations, or other modifications which, as a whole, represent an original work of authorship, is a “derivative work”. - http://www.law.cornell.edu/uscode/17/101.html
...To be copyrightable, a derivative work must be different enough from the original to be regarded as a "new work" or must contain a substantial amount of new material. Making minor changes or additions of little substance to a preexisting work will not qualify the work as a new version for copyright purposes. The new material must be original and copyrightable in itself. Titles, short phrases, and format, for example, are not copyrightable. - http://www.copyright.gov/circs/circ14.pdf
Now ideas are a whole other issue in their own right. For bigger businesses and programs there may even be a patent at play to consider but I'll leave that for another day as it wasn't really the subject of the OP's remarks.