Point 1: Only people with severe symptoms are likely to be diagnosed. People who just have the sniffles (or no symptoms) don't go to the doctor to begin with. The number of infected is likely several times the reported number. That also means that the death rate shows higher than it should.
Point 2: China is very likely concealing the real scope of this withing their country, and that is where a lot of the quoted data comes from. That suggests that the death rate could be higher.
Point 3: As stated, this is most dangerous for those with medical issues or above 50. Since the death rate is an overall average including young, healthy people, it could be significantly higher for at-risk groups.
(Takeaway from 1+2+3: The rate is still being determined, as there are lots of factors to consider right now, and while it is likely lower for the young, that means likely higher for others.)
Point 4: Preventing low-risk people from dying isn't why you quarantine something like this. Most people under 50 who have healthy immune systems aren't at risk of dying, no. So why bother? The problem is that they spread the disease to people who are at risk. They take it with them into hospitals, nursing homes, churches, home to parents and grandparents, and so forth, and that's when people start dying. You keep it away from the old people by keeping it away from the people that infect them.
Point 5: These procedures aren't intended to prevent a pandemic. They're intended to delay it. Delay it into a season that's less conducive to its spread, delay it while preparations and plans are made, delay it while it is studied and treatments are figured out.
Point 6: There is no point 7.
Point 7: See point 6.