Everything the mc did when he was sleep walking was a way of foreshadowing. He attempted to kill guwon and also attempted to slice his finger off. It all connects. I assume the finger stabbing part and killing guwon part is a recurrent event in the loops. My question is, what/who is the boulder, what/ who is sisyphus?
Yyuuup. His sleep walking episodes, his dreams, everything was foreshadowing. Which is just incredible imo.
I get where your questions come from, but there's a lot of connections to be made to this manhwa and the myth of Sisyphus besides the whole 'eternal punishment thing' (which is arguably a pretty surface level part of it).
I'd argue that there isn't necessarily a boulder, not as a tangible element at least. But if I were to guess what it is, I'd say it that, atp, it would Jinwoo trying to find a way to save the Town, as it's heavily implied that he's repeated this cycle over & over.
And, to me, Guwon is Sisyphus. Bc making that connection was part of what formed my theory in the first place.
Sisyphus was the founder & King of Ephyra//Guwon was the 'founder' & leader of the town-as a simple example of the allusion.
Another example would be how Sisyphus killed any visitors that came upon Ephyra//Guwon kills anyone that can't be of use to him & the Town.
And Jinwoo is Merope (Sisyphus' wife) in the way that, like Merope with Sisyphus, Jinwoo was willing to grant Guwon's last wish. Though I currently can't draw any other connections between the two.

Righttt, so I'd just made a new theory about this stuff yesterday and I'm willing to argue that this new chapter affirms at least some of it.
Not going to put the full thing here but, in summary, my theory is that Guwon is some sort of divine being or spirit of (or from) the Moon and that his coming to Earth directly or indirectly caused the whole apocalypse and created the time loop.
Why I think this chapter affirms my theory is because of what Jinwoo did.
He'd stabbed himself in the finger that he wore Guwon's ring on and, obviously, blood got on the ring. He'd proceeded to essentially make a wish to 'whatever God is out there' and then rushed off to go 'sky diving' and the time loop reset.
Basically, what I'm saying is that Jinwoo more or less accidentally(??) pulled off some blood magic shit and it actually worked possibly because of Guwon's ring.
This does, however, contradict another part of the theory, which is that Guwon had made the loop since he'd seemed to understand it sm in chapter 43 but this points more to Jinwoo being the one to have made it. But I would still argue that Guwon technically made it indirectly since I'm opting to believe that ring 'facilitated' Jinwoo's wish (and the ring being the facilitator could also explain how Guwon could remember/understand the loop).
Anywho, general thoughts on this chapter:
Damn, she killed them kids. The only ones that I truly feel bad for in this situation. Out of everyone, Woojun, Soo-Ah, and Soo-Ha deserved that the least. Heck, they didn't deserve it at all =(
I had half expected Jinwoo to try and follow along the original 'script' and maybe just try to alter events from the inside, but it does make a lot more sense for his character to want to prevent himself from influencing anything in the first place.
Guwon's reaction, to me, also sort of enforces another part of my above theory, which was that Guwon was consistently drawn to Jinwoo (even w/o remembering) & took him in (when he otherwise wouldn't have) because of the time loop and the fact that he created it. Also, it's just kind of funny to me. He just slapped you, bro; why you smiling?? You still down bad & you don't even remember??? Fucking dog. Neuter him.
Anddd, we all agree that Guwon's going to take him anyway, right? Jinwoo might try (and albeit fail in his current condition) to resist or fight him off, but smth tells me Guwon's just finna laugh it off and snatch him up anyway.