It’s pretty much impossible to kill yourself via Tylenol PM, not to mention that she left it, along with the alcohol she purchased, in the Saturn after she abandoned it. Suicide becomes increasingly unlikely when you consider that she didn’t have the proper means and no body was ever found.
It is quite easy to kill yourself with Tylenol PM. If enough is ingested, you wouldn't even need to pair it with alcohol. Tylenol in itself will cause acute liver failure with a high enough dose. Diphenhydramine, otherwise known as Benedryl, is the second component of Tylenol PM and is a potent sedative and antihistamine, which if taken in large doses would most certainly cause respiratory depression/arrest.