The documentation says that the family, type and proto attributes correspond to the constructor arguments. Although it is unfortunate and quirky, I think your behaviour does match the documentation.

Do the mismatched settings cause any serious problems with socket methods, or just affect the Python-level attributes and repr()?

Even without using fileno=..., you could argue that the proto attribute is not ideal:
>>> s = socket()
>>> s.proto
0
>>> s.getsockopt(SOL_SOCKET, SO_PROTOCOL)
6
>>> IPPROTO_TCP
6

Perhaps the way forward is to deprecate fileno=... in favour of Neil’s fromfd2() function. That would avoid any confusion about conflicting socket() constructor arguments and defaults. I.e. what does socket(AF_INET, SOCK_STREAM, fileno=unix_datagram_fd) mean, and is it equivalent to socket(filno=unix_datagram_fd)?