If you get a new etag with new content that's the normal and expected use of etags. They could be tracking, or they could simply have updated their content since you first loaded it.
True, but since the secondary background probing would be started immediately after page loading has completed, there is only a very short period during which this could happen. So it would only happen in rare cases. The possibility that it could happen is why I would only disable caching for that site temporarily.
There can be dynamic content, which changes every time you access it. Such content shouldn't be send with an ETag in the first place. It is however an obvious candidate for use in tracking. It is easier to implement, if you don't have to worry about interaction with legitimate use.