It is an interesting approach and location. Still, I would opt for leaving the helmet's padding intact myself. You never know what part is going to hit, it is best to try to protect everything to the best of your ability. I wouldn't be surprise to see the kiter helmet coverage on the back of the neck go lower in time.
Guess how many knives some cave divers carry using only one line that usually is taught and not attached to something that can lift a car?
Some carry up to five knives, seems like a lot but when you get trussed up, it could make the difference in getting out or not. In some cases we simply won't have time to cut. In others we may have time but won't be able to reach the single knife or may drop it, something that is easy to do. Carrying a couple of knives, attached by breakaway attachments (to avoid causing the Corkscrew of Death
makes sense. Although, sometimes just one knife can make all the difference in the world as Alex mentioned. More here