Yes, inside each course in Moodle, the instructor can add an "LTI External Tool" with all the necessary parameters for each assignment individually -- just as you described for D2L. So, the answer to all your questions int he last paragraph is 'yes'.
Previously, the Moodle admin would have created such an External Tool (where, e.g., the specific 'consumer key' of the WeBWorK server would go), and a lecturer would then select that tool and add the specific URL to the homework set or quiz to it (and would not have to care about the 'consumer key' etc. that ensure the communication between WeBWorK and Moodle via LTI). Now, the Moodle admin could still create such an External Tool but it would not be of much use since the instructor cannot specify the particular homework set. The code snippet in my previous post brings back this option of specify the particular homework set using its full URL -- at least for the current Moodle release.
Previously, the Moodle admin would have created such an External Tool (where, e.g., the specific 'consumer key' of the WeBWorK server would go), and a lecturer would then select that tool and add the specific URL to the homework set or quiz to it (and would not have to care about the 'consumer key' etc. that ensure the communication between WeBWorK and Moodle via LTI). Now, the Moodle admin could still create such an External Tool but it would not be of much use since the instructor cannot specify the particular homework set. The code snippet in my previous post brings back this option of specify the particular homework set using its full URL -- at least for the current Moodle release.