Great set of questions
A good deal of the videos that are demonstrating chain punches come from either WT or a type of WC that has incorporated it into their system. I was taught by my first instructor, "When in doubt, chain punch out." At the time this seemed like a great idea and was very effect against the untrained. If you take a close look at the stance, an individual that chain punches ends up redirecting the energy backwards causing them to move from a 50/50 stance to more of a 30/70 stance. If they can fight to stay up right, it will cause the elbows to kick out just to keep the chain going. From the get go energy is being lost. It conflicts with the efficiency concept. Also, in order to do a series of chain punches, you are required to stop the arm's motion prior to its full extension. It requires the use of the tricep to stop the arm which causes the practitioner to flex at least twice a second or even more depending on how fast the guy thinks he is
Another efficiency issue.
Chain punches look pretty cool when done with temporary accuracy and bursts of speed. This usually gives a nice blur and adds a little hollywood effect to the video. Because the videos aren't SO boring, people place them online. A good solid WC punch has no dress up to it and doesn't draw attention...Other then when you put out candles with it or flick your cigarette ash using the one incher
All the other techniques are still there but not used as much because WC wasn't properly drilled into the student's head. I stated prior that some students misinterpret the lesson behind timing and sensitivity drills and believe that the purpose is to build a certain type of "combo flow", which it's not. It's not good when this happens because it sends the student down a misleading path and will hinder the speed at which they learn WC.
I don't believe that we have moves specific to the leg trap but with WC you can take a simple move and make it do anything for you. Just depends on how you work it. I wouldn't suggest trapping the legs because it's so much easier to just pop the guy in the face, chest or neck.
WCer's don't really spar, we play around with other students, train or use it. The answer to your question is no. The head tilts back because of fear and causes the individual to lose their center and structure. They're already in the position to fall back. If I we're the opponent, I would simply step in and push their chest. My structure would cause them to topple over. The head must remain in the position which best suits your structure and the chin tucked in. Don't worry about being struck in the face. You'll forget it even happened if you win the fight. If you get put in the situation to where you think your face is going to get struck, mmmm block it? hehe
Cheers