You can't use the US population as a whole in your calculation since covid is not currently uniformly distributed. There are a lot of people who are not infected, and in fact have not been exposed or tested. Sure they are asymptomatic. Where did you get the 50% from? A variety of numbers have been bandied about. Depends on specific sample I would think.Not at all. If anything it is way off the other way.
We KNOW that over 50% never present any symptoms at all (and this is only on the ones they could test, most that had no symptoms would have no reason to get tested). So the number that have "had covid" is much higher. Making the surviving rate that much higher as well.