My question is, if Magnussen already had "pages" worth of pressure points applicable to Sherlock (evidenced by the rapid scrolling visual on his glasses), did he have to include so many other people--or is it explicable merely with "arrogance"?
I don't think he had really had that many pressure points on Sherlock.
Lets revisit what Magnussen lists in his mind:
There are only six points being listed here (and infinitely scrolled, likely for effect, to indicate that there are more than usual):
- Irene Adler (see File)
- Jim Moriarty (see File)
- Redbeard (see File)
- Hounds of the Baskerville
- John Watson
Irene Adler and Moriarty are (supposedly) dead, so unless he knows about them being alive, using them will be hard.
From Sherlock's mindpalace sequence we know that Redbeard was his dog as a kid. He got put down, so also not really a good leverage.
Magnussen says later that he did not buy the Opium/drug story from the start, so that is off as well.
I really don't know what the "Hounds of the Baskerville" pressure point is supposed to be.
So that leaves John Watson as the easiest and most likely way to blackmail Sherlock. As others have stated, he tested this earlier by getting Watson into danger and checking how far Sherlock would go for him.