> 2. Not shipping foo.py does not change the answer to #1; you still depend on it, thus your work still counts as a derivative.

I don't know whether your answers are correct but you should not use the term "derivative" or "derivative work". It's a rather clear legal term. The GPL3 does not use that term, anymore.

In fact, if we would be arguing about a derivative work in the copyright sense, your answer to 2 and 3 (and depending on the circumstances also 1) would be wrong (with the usual disclaimer IANAL).

A software B that uses a library A by "loose" linking does not constitute a derivative work. Courts in the US have develop a test for that: First, B is a complement to A. Second, it doesn't distribute any of A's code when distributing B. Third, thinking otherwise would chill the market, since no library would be used by anyone anymore, for this would make using a library an infringement of the library owner's copyright.

For further information, see:

http://www.law.washington.edu/lct/swp/Law/derivative.html