"Also, what's the point of a moving mag unit? If we haved a fixed magnitude unit, wouldn't we create the target amount of DPOR coins (on a long-term basis)?"

Let me give you a (very simplified) example to explain why this exists. Say there are two researchers A and B. A has a magnitude of 100 and B has a magnitude of 1000. A stakes once a week and B stakes daily.

With a fixed mag unit (say 0.4) then A creates 280 coins a week and B creates 2800 a week (400 per day). A total of 3080 coins are created each week.

So the creation is as follows:

Week 1:

Day 1: A--0 B--400
Day 2: A--0 B--400
Day 3: A--0 B--400
Day 4: A--0 B--400
Day 5: A--0 B--400
Day 6: A--0 B--400
Day 7: A--280 B--400

So let's say on Day 8 A has added some computational power and now A has 200 mag and B has 900 mag, but there's a problem and researcher A's magnitude is missing from the superblock for a few days causing research B's magnitude to go up to 1100

Week 2:

Day 1: A--0 B--360
Day 2: A--0 B--360
Day 3: A--0 B--360
Superblock Problem
Day 4: A--0 B--440
Day 5: A--0 B--440
Day 6: A--0 B--440
Superblock Fixed
Day 7: A--560 B--360

In this scenario more than 3080 (almost 10% more at 3320) coins are created. The fixed mag unit cannot compensate for this scenario, but a moving one can.