Third party clients that exist solely to display a certain site in a slightly different way are definitely derivative works of that site. That's not really an "if" question. I'm not aware of any disagreement by any court on this front, but feel free to cite cases if you've got them.
This was even settled back in the days of iframing/etc.
As for the rest, it turns out this particular argument has also already played out many times and reached what appears to be a fixed point - there is actually lots of caselaw on the snippets they use and how far they can/can't go, and for what purposes, before they would have to license them.
This is also, to be quite honest, not a great legal argument. It helps a lot to realize the law is not logical, it just is. Particularly when it comes to copyright, which is a very weird bundle of rights applied in sometimes very specific and odd ways to different mediums.
Things like copyright and how it applies end up very much the sum of the n thousand cases that apply it.
While sometimes you get principles out of appeals courts or the supreme court, it's relatively rare in the scheme of things (IE happens once every 10000 cases or whatever), and it's even more rare that you get generally applicable principles that you can easily apply to a new situation.
This is very different than lots of other areas of law.
Some of this is an artifact of the fact that current copyright law was still mostly built for literary works and music, and concepts don't always have an easy/obvious translation to other kinds of works (so it took hundreds of cases for courts to reach a fixed point)
This was even settled back in the days of iframing/etc.
As for the rest, it turns out this particular argument has also already played out many times and reached what appears to be a fixed point - there is actually lots of caselaw on the snippets they use and how far they can/can't go, and for what purposes, before they would have to license them.
This is also, to be quite honest, not a great legal argument. It helps a lot to realize the law is not logical, it just is. Particularly when it comes to copyright, which is a very weird bundle of rights applied in sometimes very specific and odd ways to different mediums. Things like copyright and how it applies end up very much the sum of the n thousand cases that apply it.
While sometimes you get principles out of appeals courts or the supreme court, it's relatively rare in the scheme of things (IE happens once every 10000 cases or whatever), and it's even more rare that you get generally applicable principles that you can easily apply to a new situation.
This is very different than lots of other areas of law.
Some of this is an artifact of the fact that current copyright law was still mostly built for literary works and music, and concepts don't always have an easy/obvious translation to other kinds of works (so it took hundreds of cases for courts to reach a fixed point)