2. It's up to you to choose. Either one will do. The latest price is the transaction price at that time and must be one of the buying and selling prices.
3. I suggest that you buy and sell active months, and there will be no problem, because every contract will have an active main force or a secondary main force, and everyone's transactions will be concentrated in these two months. This happens in inactive months.
4. same as above. If you choose an active month, the problem you mentioned will not appear. As long as it is the opening time, you can trade at any time. No need to wait
To answer your supplementary question, 40 lots are sold, 10 lots are to be sold, and the turnover in the main month will be very large. But if you have the strength to make hundreds of hands like this, that's another matter.