"If I derive or adapt material offered under a Creative Commons license, which CC license(s) can I use?
If you make adaptations of material under a CC license (i.e. "remix"), the original CC license always applies to the material you are adapting even once adapted. The license you may choose for your own contribution (called your "adapter's license") depends on which license applies to the original material. Recipients of the adaptation must comply with both the CC license on the original and your adapter’s license."
So here's how things would play out in the Alice, Bob and Mallory scenario you came up with:
1. Alice writes an article, publishes it under BY.
2. Bob adapts the article, gives proper attribution to Alice, and decides to license his contributions under CC0.
3. Mallory adapts Bob's article. Alice's original content is still covered by the BY license, so Mallory must give proper attribution to Alice. Bob's additions were CC0-licensed and have no such requirement.
>> Have
>> you ever asked somebody at CC if your interpretation is correct?
>
> The licenses, the human-readable summaries, and the CC FAQ all seem to
> me to be consistent with my interpretation.
Good discussion, but I suppose it will take somebody with access to a lawyer to tell us who is correct, or at least who is more likely to be correct (since as you point out it's not really settled until some court ruling comes out of it).