What is the level of impact of the callback problem?  Of the 4 scenarios in [1], it seems to me like #1 (C callbacks w/o a module reference) would be the most common.  However, can't that be addressed by adjusting the API, so it would only be a big problem in the case of a public C-API for the module (where backward-compatibility is a concern)?  #3 (classes have no reference to the module) sounds like the most problematic but how common a problem is it practice?

It may be worth including an explicit note in the multi-phase section on the scenarios that currently don't lend themselves well to multi-phase initialization.  It would also be great to indicate how to work around those issues (or the cases where for now it's better to just use single-phase initialization).

FWIW, the matter ties in directly to the work I'm doing with subinterpreters right now.

[1] https://mail.python.org/pipermail/import-sig/2015-April/000959.html