1. The late emperor, just before his death, writes the edict commaning ML's brother's army to return and safeguard the throne, namely his successor, the crown prince.

2. Before ML's brother can arrive, the emperor dies and the empress dowager stages a coup against the crown prince and burns his whole palace down. She wants her own son to sit on the throne.

3. ML's brother arrives back to the capital with his army, learns of the situation and decides to hide the edict instead of showing it publicly.

Reasons are unclear, but the most probable one:

He gets afraid the edict will be deemed fake and his army alone won't be able to defend the crown prince's faction. So he stashes it, hoping another army, led by duke Hua, will arrive shortly and that duke Hua will be able to authenticate the edict.

4. Since ML's brother doesn't have the edict on him, he and his whole army get arrested.

5. During his time in prison, FL's mother comes to him with her bleeding heart, asking him to give her the edict. FL's mother is certain that her father, the headmaster, and the scholars will come down from the mountain as the Lishan faction, and with their authority and respect held in court will be able to vouch that the edict is genuine.

But they never come down from the mountain. Instead, in protest of the political upheaval and current regime, they announce they're closing their doors and are not getting involved. The empress dowager gets angry with their decision and orders an army to be stationed at the base of the mountain, not allowing anyone to get in or out. Her order still stands even after 8 years.

Why ML's brother decided to accept FL's mother's offer instead of proceeding with his initial plan is unknown, probably was just being a dumbass.

6. FL's mother takes the edict with her to her husband's house to wait for the Lishan faction. But her husband's shitty family gets wind of her having it and, not wanting to be implicated with having ties to Lishan faction (their protest is already publicly known), they force the edict out of FL's mother, torture her and then eventually kill her with FL being the sole witness.

All in all, FL's mother was dumb af and it's majorly her fault the edict got lost.

7. FL's father gets tricked by the consort dowager to give her the edict. She takes it as a potential leverage for herself if the need arises against the empress dowager and the marquis. She forges the fake edict and gives it to marquis. Marquis destroys the fake edict.

7. Since the edict is lost, ML's brother and his men are tried by the corrupt marquis and executed on charges of treason for abandoning their post and entering the capital without persmission.

8. Duke Hua's army doesn't arrive in time, they are fighting off the rival country's army and ML is in his army at the time. ML gets crippled in that battle and then moved to the capital for his own trial. But since he was in a different army, in the end he is proclaimed innocent of his brother's crimes and gets released, but his family's name is tarnished.

9. FL witnesses everything about the edict and her mother's mistreatement in her father's mansion. Suffering from guilt over what happened to ML's family because of her own family, she harbors plans to help ML. After marrying him, she gets wind of who could have taken the edict and then successfully concocts a scheme to trick the edict out of the consort dowager.

10. FL then presents the edict publicly in court and it gets authenticated as genuine. Since there are many witnesses, the empress dowager has no other way but to exonerate ML's brother and his army.