Spoiler - Satisfaying ending explained. How to escape quicksand?

- How did ML survive in quicksand?  
ML said that the BOA agents train in the desert and are able to survive quicksand.

Moreover quicksand are not deadly in real life.
https://www.britannica.com/story/how-deadly-is-quicksand
"Nope. Quicksand—that is, sand that behaves as a liquid because it is saturated with water—can be a mucky nuisance, but it’s basically impossible to die in the way that is depicted in movies."

- How 2FL survived?
I agree with you. She is a mass murderer. She should have died after for crime against humanity.

- What happen with the land of Ruo Wen?  
Ruo Wen did not own the den he was in. So after everybody was killed and the premise damaged, they just abandonned it. Maybe another gang of criminals relocated in it because it was not destroyed.

- Why Ruo Wen had to die?
1) Ruo Wen was a mass murderer so he had to die.
2) His life's purpose was to save ML's life. Now that the truth was revealed, he had to die. Moreover, he never regreted his actions and he wasn't a good guy.

- Past of the villian?
We already know Ruo Wen's past. I don't think we could know all his gang men's past.

- The secret of the book?
The" desert scroll" is supposed to help desert's inhabitant the live a better life by avoiding desertification and managing the water.
It was never a tool of power. However, bad people like Na Zhan wanted to weaponize it.
The "desert scroll" is important for survival. But it not a tool of power.

If you do find yourself stuck in quicksand, the best idea is to lean back so that the weight of your body is distributed over a wider area. 

Moving won’t cause you to sink. In fact, slow back-and-forth movements can actually let water into the cavity around a trapped limb, loosening the quicksand’s hold. Getting out will take a while, though. 

Physicists have calculated that the force required to extract your foot from quicksand at a rate of one centimeter per second is roughly equal to the force needed to lift a medium-sized car. 

One genuine danger is that a person who is immobilized in quicksand could be engulfed and drowned by an incoming tide—quicksands often occur in tidal areas—but even these types of accidents are very rare.