Sorry, all you get is what you see in document.cookie. The cookie metadata like path, domain and expires are not visible to site code (neither to JavaScript nor to the server-side).
To read a cookie that is being shadowed by a cookie with a more-specific domain or path, the only thing you can do is load a page for which the more-specific cookie is out-of-scope, and read it from there.
If, as you say, you only need to remove a cookie, what you could do is try to remove the cookie at every possible level of specificity, eg.:
document.cookie="foo=;domain=sub.domain.example.com;expires=Sat, 01-Jan-2000 00:00:00 GMT";
document.cookie="foo=;domain=domain.example.com;expires=Sat, 01-Jan-2000 00:00:00 GMT";
document.cookie="foo=;domain=example.com;expires=Sat, 01-Jan-2000 00:00:00 GMT";
and similarly with the path variable. You could put this in a nested loop for each path and domain part, splitting on . for the domain and / for the path.