Tree with Deep Roots Episode 4

Cluedo

It's official: I'm stupid.
Basically, I've understood nothing of this episode, except:

1. We are finally out of a flashback that lasted 3 episodes and a half.

2. Grey bearded, wrinkled fogeys in hanbok will never win a word battle against young scholars. Because these last are prettier.

3. While severing enemies heads and happily basking in bloody battles, Ddol Bok found the time to learn how to write

4. Korean eat seal penis (???)

5. There's a corpse in the library, but the murderer wasn't Professor Plum with the candlestick.