Well, the question of an active participation of the UN in security, even for stabilisation, is a pure theoretical one. The South Sudan mission is under chapter 6, with the possibility if there is huge deterrence of the situation to use chap 7 privileges.
But the SRSG reaffirmed that up to now and for the future, UNMIS would implement a full chap 6. In clear a wait and see UN mission. The other problem is manpower. No one is really ready to provide more troops for SS.
Also, if things go bad, UN will redraw. And that is what the oldschool UN South Sudan wants: to go back to Naerobi and its confort (we are far from people centric concerns). Basically if violence errupt, UN loose their mandate...

So UN tried to push, and is still trying, SPLA to intervene into cattle wars. But as said previously, there are numerous question marks on the role of SPLA into starting the clashes. For me, it looks like calling the incendiary guy to play fireman.
On the other side, GoSS is trying hard (they say) to show they do have control. But… If they are not as bad as FARDC, they not too far from them in terms of discipline. Sometime, I even wonder what they have in mind. They contracted Kenyan army to train them. I have pretty low knowledge of Kenyans army records, but still, SAF in Khartoum must be laughing.