Extremely insightful interpretation!! You understand the characters + context surrounding the world and how it shapes the characters beautifully.
I might even go as far as say that Pyeonghwa is also a sadist since he knows Incheol does not want to hit him and yet Pyeonghwa does not give me another way to deal with him. He almost revels in the way he does not give him another choice. He likes to see Incheol's mixed feelings on the matter
With the context of Chapter 24, it does seem like Pyeonghwa is projecting a lot of his trauma surrounding the policeman on Incheol. He didn't have power as a child, but he does now. He clearly isn't processing his emotions with respect to his sexual assault in a productive or healthy manner, and it seems to be manifesting, as you said, in sadistic & controlling tendencies towards Incheol. Forcing Incheol to hurt him, but only on his terms.
This is so good. The S/M interpersonal relationship between Incheol and Pyeonghwa is the determining factor for how this huge, politically important redevelopment project will go. Their relationship is one where the clear-cut class divide blurs. Pyeonghwa being young, and with no social influence vs Incheol who is older, likely richer, and with a lot more social power. In any other circumstance Pyeonghwa would be nothing but a minor nuisance. That's pretty much the premise for why Incheol was sent in. But meeting Pyeonghwa leads Incheol to re-evaluate his place in the Assemblywoman's hierarchy of aides. Pyeonghwa is alone and lacks social clout but he holds the deed for the land, his biggest source of power.
This blurring of power dynamics is explified in their sexual dynamics. Incheol being submissive, and having to lean into sadistic tendencies to hurt Pyeonghwa who is a more dominating masochist. Stereotypically, in BL media at least, the dom and sadist are considered to be in the same box(same for sub and masochist). These stereotypes and social conventions break down when they are interacting, thus highlighting the fragility of their positions.
It's such a good commentary on class and exploitation. Amazing stuff.