It depends on whether or not there was negligence and also on state laws. In your skydiving example, if the main chute failed but the backup chute was fully functional and you didn't do what you were supposed to in order to deploy it (I've never done skydiving so I don't know if this is automatic or not) then they won't be liable because you accepted the risks of the activity. However, if the entire backpack rips off because the straps were half cut through, then they will be liable no matter what you signed because they were negligent.
For the driving experiences, if you sign the waiver (you have to) and get seriously injured because you crashed into the wall they won't be liable because crashing is an inherent risk to the sport. However, if the injuries are due to the 5 point harness failing during the crash or having not been properly checked before you started then they will be liable.
I know from skiing in Colorado that there are state laws that make it so that you can't hold a ski resort liable for anything. If you ski into a lift support pole, you can't say that it shouldn't have been there or there wasn't adequate warning. By state law you accept all risk when you decide to ski.
In this particular case, Petty Holdings (and possibly Disney because of our ridiculous legal system) will certainly be liable for the death. With an employee there aren't any waivers anyway but if it was the guest that was killed, there would definitely be liability because they were negligent in the use of the track in the wrong direction. However, if the driver tried to sue over his injuries (if he had any), he'd be unlikely to win because crashing was an inherent risk of the activity and he signed a waiver. The driver would have to prove that running the wrong way made it more likely to lose control or that somehow his injuries were caused by the guard rail impaling the instructor.
Of course, no matter what is signed, anybody can always file suit about anything. The waiver would be used as a defense. Due to the cost of fighting suits, it is likely that a company (or, more correctly, their insurance company) will settle because it will be cheaper than going to court and fighting for 5 years.