This is all about admin level site settings for grade items and categories.
Current grade item settings allow you to set overrideable defaults but not force these.
Current grade category settings allow you to force certain defaults without overrides.
These should be rationalised and cleaned up so that:
1) Each setting defines a default (the default default is defined as now in moodle/admin/settings).
2) An additional checkbox for each one allows the admin to turn on "Force" for that settings, which means that teachers can't change them in their courses/items.
The forms themselves are not too hard to change, but will require a lot of renaming probably.
The grade forms will need to suport these new settings so that moodleforms locks the fields appropriately.
The code itself needs to be very carefully checked for each setting to make sure all the behaviour works as before with no regressions (don't forget ouuser report!!). This part is probably quite hard work. Create some subtasks so we can help if necessary.