Assignment and Calendar should more consistently and accurately display the Due dates for teachers of multiple groups when group overrides are applied

Scenario:
1. Teacher runs a course for 2 or more groups (let’s assume Group 1 and Group 2 here).
2. Teacher has set an Assignment, adding an overall Due date.
3.The Assignment uses the group mode of “Separate groups” (though the big described affects any group mode, in fact).
4. Teacher has added group overrides with a different due date for each group.
(When adding / editing an override, there is a reset option “Revert to assignment defaults”, suggesting the Due date inside the assignment is a “default” – a term that could conveniently be used more broadly as needed.)

 
Encountered behaviour:
1. When visiting the Assignment, the teacher can switch between the groups, and view the changing overridden Due dates – Good.
HOWEVER:
2. The (default) “Due date” displayed on the Announcement page when the group filter is in the “All participants” state is NOT the Assignment’s default but the group override value with the highest priority. This is ILLOGICAL and CONFUSING.
3. The same highest-priority override Due date (and only this one) is displayed in the (teacher’s) Calendar . 
4. Additionally, it’s confusing that if we remove the Assignment’s “default” Due date from the settings, and only keep the overridden dates, the calendar will first stop listing any due date at all; the override must be opened and re-saved again for the Calendar to pick up the date.

Cf. attachment:

  
Expected behaviour:
1. When visiting the Assignment with the filter det for “All Participants” – the default Due date set in the Assignment settings should be displayed. A notice that this a default Due date should be included – especially if any overrides are used. 
2. IN ADDITION, a warning that that (Due date) overrides exist for groups (and / or for users if applied) should be very clearly stated – as indicated in this earlier tracker: https://tracker.moodle.org/browse/MDL-73922 .
(This would encourage the teacher to switch the group filter and check for non-default due dates, and/or else to check the group / user overrides pages.)
3. The teacher’s calendar should list ALL these Due dates:
•    Default due date (applying to students that do not belong to Group 1 or Group 2)
•    Group 1 Due date
•    Group 2 Due date
This should help the teacher see and manage the overrides effectively.