Correction to 7 ways to reach base without a hit:
The author is correct that this is a classic brain-teaser, but traditionally the 7th answer is "Obstruction" (not "Pinch Runner").
1. (Why obstruction should be included) Obstruction is different from catcher interference (which you listed as #5). Catcher interference is when the ball is out of play and the catcher illegally interfere's with the batter's swing, whereas obstruction occurs AFTER the ball is in play and an umpire or fielder illegally blocks the base path of the runner. Obstruction is another way to reach first base without a hit.
2. (Why "Pinch Runner" should be excluded) Remember that we're talking about how to reach first base WITHOUT a hit. But to come in as a pinch runner, you're replacing somebody who already got a hit (or reached first base on one of the other seven methods we already mentioned).