you are right

'如果2005年我若盈利的话，是不是可以把剩下的抵消，比如若2005年盈利5.7万的话，我到时的capital gain就是0？' YES. Usually long-term lost 抵消 long-term gain first, the left then can 抵消 short-term lost. Same thing is true for short-term lost. Short-term lost 抵消short-term gain first. The left then can be used to 抵消 long-term gain.

'我用margin account的利息2千多就不能item deduct了而只能carry over到以后年份了？' YES. It can only cover the income you make in the market, either from stock or interests you make in your investment account.

These are my understanding.