1.香港的office未有在香港註冊 and runnig the business in Hong Kong, it is illegal for business operation ! Make sure your company has applied th Business Registration !
Although your employment contract is signed in USA and paid the salaries in USA, your working is still in Hong Kong. You are a Hong Kong permanent resident and you cannot apply the 60-days rule as a visitor !
You need to pay the Hong Kong salaries tax!
3. 另外報稅表要在一個月之內寄回, 是計到稅局手上定郵戳日期??
Yes. According to the date chop that the IRD received your annual salaries return.