
This is a continuation of Kwon's backstory, to recap: he's a bastard child who basically got exiled by his father to go defend the border after his stepmother had a mental breakdown and tried to kill him over his natural ability to excel in everything that overshadowed her son.
Though he was meant to just die on the frontline, his ruthlessness and willingness to ignore authority to get a more favorable outcome caught the attention of the king's younger brother who offered him the titles position/rank of an officer. With no other choice he accepted and ofc excelled in the role. Now the king's brother is going to cash in on the favor of making him an officer to which he won't be able to refuse. It will soon tie into the present storyline. Hope this helps!

The king in the flashback is the father of the current king. The grand prince is the former king’s brother and therefore is the current king’s uncle. In the time period between Kwon’s promotion and the grand prince reminding him of his debt to him, the former king died, causing his young son to ascend to the throne and making the child the current king.

This whole arc isn't vibing at all. None of the characters are really sticking to their established personalities, the dialogue is weird, the MC's action makes no sense, Ben who was probably one of the most interesting characters is just being reduced to boring stereotypes and the whole plot feels like it got thrown out because the author just wanted to have some kind of a sex scene but the chemistry just isn't there.

Satisfying slap aside, I really hate Poato, he was checking the supplies with doc dan, they even specifically sprayed the pain relief spray and commented about it, and Potato said absolutely nothing and didn't come to Dan's rescue when the spray injury happened and before the match commenced. Literally this whole issue and everything that came from it could have been solved/investigated had he spoken up. Now they're trying to redeem him?
As a refresher, Mu'er first shows up in chapter 15 to help Parker and the search party find Bai after she's taken by Curtis. Then he gets bitten and poisoned by Curtis in chapter 18. In chapter 19 Bai takes the scale from the tip of Curtis' tail which is the antidote to the venom.