If you set the deadlines at the course level, all users whether enrolled by and admin or self-enrolled will have the same “due date”. Only way I know to change this is to do it manually. You have to find the users and then change their active enrolment period or remove the deadline all together.

I did see that the new enrolment attributes should be available via the enrolments rules soon (you can see it in sandboxes now) where you can assign courses to a group of users but set the enrolment level to optional, mandatory, required or recommended…

Not sure if that would help as you would need to assign everyone who is not required to complete the course(s)

The other option would to be have 2 courses; 1 not in any catalogue that is assigned with due dates and the other open and in catalogues for self-enrolment.

Hope you find something useful here.

I would only add that if you take option 2 mentioned by Irnlab, I would configure equivalency settings so that ABC users still get credit for the “hidden” course if they take the “public” one.

Only thing I don't like about equivalencies is that BOTH courses appear on the user learning history and besides the “time spent in the course” value, there is no way to differentiate them; they have the same completion status