Addressing your point (5):

> (5) dbm.gnu is not a default, and I can't even work out how to get it enabled with the stock Python installation (i.e. without building my own Python against e.g. Macports gdbm)

If you are using MacPorts, the easiest way is to use a Python from MacPorts. For example,
  port install py36-gdbm
would install everything you would need to use gdbm with their python3.6.
  
> Is it a problem to ship dbm.gnu as part of the default install, so that it is more feasible to assume its existence?

The main problem is that gdbm is GPL3 licensed.  Python source distributions do not include any GPL3-licensed software to avoid tainting Python itself.  We therefore avoid shipping GPL3 software with python.org binary releases, like our macOS installers.