My problem with these internet demonstration videos is that they always feature at least semi-willing accomplices, who know that the purpose of the video is to demonstrate how effective the technique being used actually is.
That being said, the techniques shown can be effective, but they assume a couple of things:
- Proximity - The technique presumes that you are allowed to get into a position close enough to be able to actually do the technique.
- Grip and strength - The technique also assumes that your strength and surprise is enough to overcome his strength and tension, as well as your grip being strong enough to control the arm.
What happens if the knife holder is significantly stronger than you? What happens if your grip slips off during the grab? (Hint, the reflexive tightening of the forearm will drive the knife back up towards the throat, you do the math). What happens if you can't get close enough, or the knife holder moves away as you approach? What if it's raining and/or the guy is wearing thick clothing? There are too many variables to simply say "Yeah, that's effective".
For your other question, yes, there are myriad ways to approach someone, move around them, grab/control the knife hand, disarms, etc. Almost every art out there has some form of blade related defense. I'm trained in the Human Factors Research Group variants, PPCT, gun disarms, etc. The system is great (IMHO), but am I good enough to apply them in a stress situation? I don't know, never had to use it yet, thankfully.
So yes, the techniques could be effective depending on the conditions and skill of the practitioner, but like everything, it could also go horribly wrong.