Legally, you are always entitled to your own self-defence, if you are assaulted you are entitled to defend yourself with reasonable force. You only come a cropper when you make statements like "he deserved it", " I wasn't having that" or "I'm sick of this sort of thing happening, so I tw@tted him"........stick with " I was scared for my own safety" and you are OK.
As for what is reasonable force? anything that you can justify.....so no, shotgunning a burglar as he runs away is not, shooting him as he comes at you with a screwdriver would be.
And as for punching someone, I'm a boxer etc... it's asking for it. Defence is a whole lot easier with a grappling/hold technique. Just watch total fighting etc.... no boxers there, most champions are shoot-fighters (tai-kwon-do to Judo once close enough).
If you are genuinely worried about being assaulted by a punter, you need to talk to your boss, maybe look into an adequate form of self-defence..if it happens often, maybe buy in some training.
(I instruct physical intervention, but the cost for my courses would be unworkable for a paintball site. but their are others out there, mine is fully insured and the company will send expert witnesses to back you should legal action occur).
And as for whose fault a mask lift is..... you just need to prove negligence to take legal action, and we all know what disclaimers are really worth. All it takes is "well the marshalls lifted their masks", "my elastic was loose" or " Well I didn't hear them say that" and it's sh!t creek......... this is why site insurance is so high.
If someone is blinded on a site, the site may be clear and free with the HSE, but the punter will definately sue, and most likely win.
Oh, and how many times do you have players on site who haven't filled in a disclaimer at all? in the rush to kit up 30 players maybe? or a late show who arrives as the game starts?