> if NotADirectoryError should be raised, it should mention '/path/to/file' 
> rather then '/path/to/file/somename.txt'.

POSIX specifies that C open() should set errno to ENOTDIR when an existing path prefix component is neither a directory nor a symlink to a directory [1]. What you propose isn't possible to implement reliably unless the filesystem is locked or readonly, so it should be handled by the application instead of by the system or standard libraries.

> FileNotFoundError: [Errno 2] No such file or directory: 
> 'C:\\path\\chrome\\to\\file\\somename.txt'

Windows specifies that this case should fail as follows: "[i]f Link.File.FileType is not DirectoryFile, the operation MUST be failed with STATUS_OBJECT_PATH_NOT_FOUND" [2] (see Phase 6 in the linked pseudocode). 

The Windows API maps this status code to ERROR_PATH_NOT_FOUND (3), which is distinct from ERROR_FILE_NOT_FOUND (2). However, the C runtime maps both of these system error codes to POSIX ENOENT. All isn't lost, however, because it also saves the OS error in _doserrno. io.FileIO could preset _doserrno to 0, and if it's non-zero after calling _wopen, use its value with PyErr_SetExcFromWindowsErrWithFilenameObject instead of calling PyErr_SetFromErrnoWithFilenameObject.

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[1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html
[2] https://docs.microsoft.com/en-us/openspecs/windows_protocols/ms-fsa/8ada5fbe-db4e-49fd-aef6-20d54b748e40