I think you're making it more complicated than it really is. The department of education requires proof that each student is actually doing some work. That's why they set up the regulations.
This does not mean that the assignment dates in the tut letter are wrong. It just means that in order to meet the regulations, you must submit one assignment before 15th May. This assignment could be any one of the 4 that have been set by the department. Just remember that assignments submitted before the due date will probably not be marked ahead of time.
Practically though, this means that if you miss the deadline for assignment 1, then you HAVE TO submit assignment 2 or you won't be able to write the exams.
The second regulation requires that you have to have accumulated 100 credits in order to write the exam. The department has added to that by saying that we have to have earned the required credits before 14 July. What this means is that we must submit AT LEAST 3 assignments in total since maximum credits awarded are 40 per assignment.
Since this is cutting it a bit close, it would be stupid to miss handing in an assignment. Also remember that there are only 3 values of credits that are awarded: 0 for not submitting, 20 for inadequate work and 40 for adequate work. If you hand in 3 inadequate assignments, you won't be allowed to write the exams (3 x 20 = 60 credits).
To sum it all up, hand in all of your assignments and stick to the dates on page 9 of tut101 and you'll be fine - you won't have to worry about the DET regulations.
Cheers
Rob