> inet_pton() requires an address family. Should we iterate on all supported address families until one works?

No, that is not necessary. inet_aton() only accepts IPv4 addresses. Therefore an emulation with inet_pton() would look like this:

>>> import functools, socket
>>> inet_aton = functools.partial(socket.inet_pton, socket.AF_INET)
>>> inet_aton('127.0.0.1')
b'\x7f\x00\x00\x01'

> inet_pton() doesn't accept IPv4 "short format" like "127"

Yes, that might be a problem.