I think you mean the fifth century, as the Ballad of Mulan is believed to have been written during the era of Northern Wei. Some historians have tried to place it around the Han or Sui dynasties, but I've never heard of a retelling set in in 15th century China.

The Tuoba clan (who ruled during this time) were extremely mysogenistic. Tuoba Gui, founding prince of Wei, instituted a system that eventually became commonplace among his successors: the mother of the crown prince must commit suicide. Any crown prince who tried to save his mother's life would have lost the right to the throne.

This was done as a preemptive measure to ensure that the matriarchal family would never compete with the patriarchal family for power. Now, there is no record that any mother of a crown prince was ever suspected of planning to become an empress dowager. Plain and simple, the Tuoba clan hated women and wanted to ensure that no woman ever came close to power.

In this extremely mysogenist society, I wouldn't be surprised if some woman considered crossing the gender barrier as an attempt to save her life. But in so doing, she would face the problem of not having a family. After all, her parents would certainly remember whether they had a son or a daughter.

This is what makes the legend of Mulan so interesting, as she goes to war away from anyone who knows her. But still, she will run into the problem of returning home.

The Ballad of Mulan concludes with the protagonist resuming her life as a woman. This is not how her story would have really ended.

As I wrote in another answer, the legend Mulan was not intended to be a story for women to follow. Rather, it was created by men who were primarily interested in objectifying women. The story of a woman saving her father's life and then returning home to the patriarchal society from which she came would have been quite titillating.

So, what would have really happened to Mulan when she returned home?

Well, strong women were either killed or objectified. Usually both.

Ming dynasty Zhu Guozhen posed this exact hypothetical question and then answered it in a chapter titled Women Generals from his book A Miniscule Book from the Yongzhuang Studio.

Zhu writes:

When the royal court finally heard about Mulan’s true nature, Emperor Yang offered her a position in the royal harem. Again, Mulan declined, saying, “Your humble servant is unworthy of this honor.” When the emperor tried to take her by force, and she realized that she could not resist his demands, she ended her own life.

(If you've heard of this ending before, you're probably thinking about the novel Romance of Sui and Tang. The author, Chu Renhuo, borrowed this ending from Zhu Guozhen.)

So, let's suppose a woman successfully crossed the gender barrier and joined the army. From then on, she would never be able to return home. The story would end with her being a nomad for the rest of her life.

But unless she joined a clan of other nomads (which is much harder to do than the movies would like you to believe), she would not have the strength to resist bandits. So, she wouldn't last long.

No matter what happens, a woman disguising herself as a man cannot end happily.