What's to say the stock market would skyrocket? Especially if this cure is accessible to everyone and published freely online, the only direct effect is pharma companies stocks crashing because of a loss of oncology drug revenue. Who's to say what the rest of the stock market would to to react?
Even if it's a tossup, does that mean there's a 50% chance of the inventor of such a cure being liable for that crash?
It doesn't work like this. If it's a tossup (ie probability =~1:1) then there's no legal liability. Importantly, whether the cat's dead or alive is irrelevant.
The issue here is that it's not a tossup. The probability is >=2:1 hence the liability, even if that is alive after the plunge.
Even if it's a tossup, does that mean there's a 50% chance of the inventor of such a cure being liable for that crash?