Student or Learner
It's kind of confusing. In the last line, does the first "would" mean "habit" or "future in the past"? The second must mean "future in the past".
p3)The person on trial was required to open one of the two doors. He could open either door. He was given no direction or no influence, other than that of a fair chance. Behind one of the doors was a fierce tiger. If he opened that door, the tiger would spring upon him and tear him to pieces as punishment for his crime. (B) The fact that he chose that door was a sign of his guilt. Behind the other door was a beautiful lady. If he chose that door, it would be a sign of his innocence and he would be allowed to marry her, then and there. This system of criminal justice was popular among the king's subjects. When the people gathered to see a trial, they would not know that they would witness a cruel punishment or a happy wedding.
Yes, the first would is 'habit'. As things customarily occurred, the people watching would not not know what they were going to see.
"I didn't know that I would witness a cruel punishment". Yes, that could be described as relating to the future in the past.
Perhaps that's what 5jj means - why are you misinterpreting the first "would"?