> So for an actual non-root mount point ntpath.ismount() returns True 
> and with IO_REPARSE_TAG_MOUNT_POINT included ntpath.islink() also 
> returns True. nt.readlink() returns the "\\?\Volume{GUID}\" path

If islink() is true, then st_mode has S_IFLNK and not S_IFDIR. So we have a mount point that's a symlink, which is not possible in POSIX, and it's not a directory, which is unusual in POSIX to say the least. 

For cross-platform consistency, I think it's better if ismount() is a directory. The question would be how to ensure that's true in all cases. Windows make this hard to accomplish reliably due to DOS 'devices' that get reparsed in the object manager to arbitrary paths, not necessarily to volume devices.

> Root mount points ("C:\\", etc.) do not return true for islink()

Not really. Here's a mountpoint-symlink chimera:

    >>> os.readlink('C:/Mount/Windows')
    'C:\\Windows'
    >>> os.system('subst W: C:\\Mount\\Windows')
    0

It's a symlink and not a directory:

    >>> os.path.islink('W:\\')
    True
    >>> os.lstat('W:\\').st_mode & stat.S_IFDIR
    0

But it's also a mount point:

    >>> os.path.ismount('W:\\')
    True

The object manager reparses "W:" as "\\??\\C:\\Mount\\Windows", and we open it with a trailing backlash, which is fine, i.e. "\\??\\C:\\Mount\\Windows\\". 

> I'm not seeing why having both islink() and ismount() be true 
> in this case is a problem.

It's only possible if a mount point is not a directory. That we'd be returning this for a junction is a strange state of affairs because a junction must target a file system directory. I prefer generalizing junction as a name-surrogate type that allows S_IFDIR.