Yes, that should be https://webwork.yourschool.edu/webwork/ltiadvanced/content_selection in the LTI 1.1 instructions. The ltiadvantage url is for LTI 1.3.
You shouldn't set $authen{user_module} in localOverrides.conf. That should be set in authen_LTI.conf. That is unless you are using another remote authentication method (like LDAP).
We need to add to the instructions about cookies. In order to use the content selection endpoints with either version of LTI, you need to enable cross-site tracking (or allow third-party cookies depending on what your browser calls this setting). Otherwise cookies will not be set when the content selection dialog is rendered in an iframe by the LMS.
I don't recommend using $session_management_via = "key". That is not very secure.