Short Answer
How can the United Nations utilize its available options and powers to prevent further escalation and slipping into the abyss, and what are those options?
All the U.N. can do is encourage the interested parties to talk. As a practical matter, there is really nothing else it can do. Any U.N. Security Council resolution would be vetoed by one or more of the permanent members.
What measures can the UN take to avert regional war in the Middle East?
None. That boat has sailed. There are multiple regional wars already in progress in the Middle East.
Frame Challenge
Also, despite the dire language in the question, it is not at all a given that de-escalation or preventing further armed conflict is a desirable end.
There is an emerging school of thought in the foreign affairs world that de-escalation and cease fires excessively prolong conflicts that might otherwise end with a decisive victory for one side or the other that would end the conflict, reducing the harm caused by it in the long run. The conflict between Israel and the Iranian proxy Hezbollah has been grinding away as a low level armed conflict for decades. Intervening to stop the conflict now could prevent it from coming to a decisive end that could bring about long term peace.
One of the leading academic articles making that case is Edward N. Luttwak, "Give War A Chance" 78(4) Foreign Affairs 36-44 (1999). Its abstract summarizes the argument as follows:
Since the establishment of the United Nations, great powers have rarely let small wars burn themselves out. Bosnia and Kosovo are the latest examples of this meddling. Conflicts are interrupted by a steady stream of cease-fires and armistices that only postpone war-induced exhaustion and let belligerents rearm and regroup. Even worse are U.N. refugee-relief operations and NGOs, which keep resentful populations festering in camps and sometimes supply both sides in armed conflicts. This well-intentioned interference only intensifies and prolongs struggles in the long run. The unpleasant truth is that war does have one useful function: it brings peace. Let it.
Ceasefires fail about 80% of the time, and are more prone to failure in some situations than others.
The case for taking this approach in the Gaza conflict has been made, for example, in a January 2024 article.
Allowing Israel to destroy a designated terrorist group that the country of Lebanon has been unable to stop from operating in its territory may be a preferable resolution, even if it means a brief period of escalation in an already decades old regional war.