The second question: Actually, there will be some tickets at the train station at 7: 00 or 8: 00 on the sixth night, so you can try your luck at this time. But as long as it is past zero, you can buy tickets within five days (if you can't sell them, you can go to other ticket outlets)
The third question: Actually, it depends on whether you have time. If you have enough time, you can go to the station to buy tickets at 0: 00 on the fifth day. It is best to book tickets by phone at five or six in the morning, which is more likely and has lower cost; If you are far from the station and can't book a phone, it is better to queue up at the ticket counter early.
Wish you an early ticket.