You're very smart, zhoutong, and your eager and polite acceptance of feedback does you great credit. But I would not attempt what you're doing, and I know a fair amount about both trading infrastructure and security.
And though your site is very impressive, I immediately spot a major omission: you say nothing about your margin call policy. Do you have one? What will you do when one of your users' accounts goes to zero, and then negative?
We have two metrics: net value and minimum net value. When NV < MNV, all positions are immediately liquidated. When NV < 2MNV, a warning is visible on trading panel. (Future feature: margin call email)
These metrics are completely transparent, showing in different colors to represent health status. Once you give it a try you will know.