These are deep questions, and I'm not a SME--just a student of them. Here are my thoughts:
1. Of course, I'm only interested in stopping a threat from continuing to do bad stuff to me or my family.
2. "Get off me, stay off me" is the main purpose of a defensive knife. That's likely to happen after a bunch of other things have already gone wrong, and the BG is already on me and I'm entangled.
3. Defang the snake is primarily a knife-on-knife technique from a face off. See above. EDIT: it can also be a good technique to keep someone from closing distance, as in a rape attempt.
4. Heavy clothing, leather jackets, blankets, etc. seem more likely to defeat an edge than a point--especially in a fucked up tangle.
5. Point-driven knife use does not have to be lethal, and has a lot of advantages. E.g. Tip up, edge in can defeat an overhand grab to the wrist.
6. I like serrated blades. They are so useful for emergency cutting. It's the main reason I own my D/E Microtech.