My old boss, Dave D., was in the research group at UW that elucidated the mechanism of acetaminophen liver toxicity. A highly reactive benzoquinone species forms. It is very hard to observe because of the reactivity. It was trapped by reacting with another molecule and it's existence inferred.
Liver failure is a real bummer. Don't mess around with Tylenol and alcohol.
Regarding Aspirin versus Naproxen. I read somewhere that Aspirin is a 4000:1 COX-1/ COX-2 inhibitor, which explains why it is so harsh on the stomach. The prostaglandins that COX-1 produces are protective of the stomach lining. Naproxen is 1:1 COX-1/COX-2. This is the lowest ratio of all the over the counter NSAIDS.