Claiming that we know Disney was not legally negligent in this case is simply a ridiculous point. We can be of the opinion that Disney was not at fault, because that involves subjectivity. But its also irrelevant to everyone but ourselves.
But onto the speculation...
Yes, the audiences for WDW and the local carnivals have some overlap. But they are still not identical. Further, anyone with a borderline constitution can see exactly what the Gravitron is all about, unlike with M:S. That doesn't necessarily make Disney negligent with regard to anything, but you can't dismiss the arguement on the grounds that they are the same ride for the same audience because they are not.
Further, when you look at the impacts ride has, all aspects have to be considered, not just the number of G's it pulls and for how long. The visual stimuli are going to have a tangible physical impact on the rider, and that is another key difference between M:S and a Gravitron. Again, that doesn't prove anybody is at fault, but it does mean the Gravitron can't be used as a reason to exonerate Disney.
Personally, I have no idea how the case will turn out. I can see it going either way. I do think that most likely Disney will settle.
What disturbs me is the way Disney was caught off-guard by the number of cases of nausea and other issues M:S caused. While certainly that does not prove legal negligence on their part with respect to the boy's death, I don't see how one cannot at least have questions about their testing with regard to this attraction.
This hasn't been mentioned lately, but the idea that it was proven that the boy was just as likely to die anywhere else as on M:S is simply not true. The boy had an undiagnosed heart condition, one that is largely asymptomatic. Meaning you may have taken your child for regular checkups to the top pediatricians in this country and still not know he has it. Unfortunately, the first indication that someone has this condition is often cardiac arrest. The arrest can be brought on by stress.
So yes, it is possible this boy would have died the next time he tried to run, or rode Big Thunder. However, it is possible that the stress induced by the physical and visual aspects of M:S was the only stress he was going to experience that was great enough to induce the arrest. Many children with the condition never go into cardiac arrest, and the risk is greatly reduced by the time they become adults.
Now, again, even if itwas the case that M:S was the only thing that was going to cause the boy to go into cardiac arrest, it wouldn't prove negligence on Disney's part. But at the same time, the coroner's report and the government inspection do not prove the death was coincidental either.
It all means that any definitive conclusions drawn at this point are based on incomplete information. There are reasons to believe that either side could prevail.