1) I'm not sure what you are asking.  Does anyone think that it is simpler to register two different callbacks than one?  IMHO it complicates the interface and creates so many oppertunities to do things incorrectly.

2)No, it is an oversight, let me verify the code.

3)I don't think we ought to be smart and try to remove callbacks.  I don't think that is common practice, the developer will know soon enough if things don't work.  Just keep it simple.

4)Interop, I think, is not an issue.  the gc module is a C-Python only implementation detail.  Every implementation has the freedom to collect garbage in its own way.

I'll post an updated version soon.