Some questions about lore and plot

I absolutely love this movie, but there are a few things that I'm confused about or just don't get. First of all, what was that finger-to-the-forehead technique the two priests used in that first scene, and why was it so effective? Could they have used that instead of the spells on yellow rice paper? Did they bite their fingers and draw their own blood? Another point of confusion was the clumsy police chief character's apparent affection for his cousin. Were the subtitles I saw correct in saying they were cousins? And was he really into that kind of thing? I wouldn't mind if he was (it makes for a good character), but I thought it was a strange plot point, so maybe not entirely accurate. The last point of confusion for me was the stiffness of the corpses. What I mean is some were stiff and some weren't. The first time we see the grandpa corpse, it's stiff and hopping around. When his son becomes a corpse, he moves stiffly, but not hopping (I assumed because he hadn't stiffened up completely yet). But when we see the corpse for the final showdown, he isn't hopping around anymore. Could this be because he's gotten stronger by absorbing the qi of others? I'm not trying to pick the movie apart or anything, I was just wondering if any of these things had any bearing on traditional Taoist beliefs.