No one was topping anyone there was no sexual intercourse but just oral! Jung is actually the first person to ever have a sexual experience with penetration with mookyul!
So I double-checked what previous repliers said and it's true, in Sangchul's Diary, Mookyul said (after Sangchul found out about the president and Mookyul kissing and getting each other off) "the old man doesn't like piercing holes in my body. In more ways than one" with a meaningful look at Sangchul.
I think we can quite reasonably infer that there was no penetration. Or at least the vast majority of their sexual relationship did not include penetration (there could have been attempts, for example, or it could have happened once)
And yes Sangchul attacked Mookyul while Mookyul was drunk, gave him a blowjob.
Hmm... I want to see Mookyul as the bottom for once. I mean I bet he was the uke when he was with that old man. I kind of feel jealous that I want Ewon to at least have a taste of that ass or else that ass was only been used and was only limited for that and by that old man only. Ugh