Can diplomatic efforts defuse Lebanon's Mideast powder keg?

Despite explosive violence in Lebanon in recent days -- and continuing predictions the country is on the brink of a Palestinian-Israeli showdown -- a flurry of diplomatic forays may intervene to preempt all-out clashes.

Three bomb attacks Feb. 23 and 24 caused an estimated 70 casualties in Beirut's predominantly Muslim and Syrian-patrolled Raouche quarter -- the most destructive incidents here since an Oct. 1 bomb blast in the predominantly Palestinian quarter. The hijacking Feb. 24 of a Kuwaiti airliner by supporters of the Shiite Muslim leader Moussa Sadr adds to the chaotic situation in the country.

Nevertheless, diplomatic efforts in the days ahead may work to keep this mountainous coastal country -- just now experiencing its first balmy spring days -- from sliding into what many have feared: a battlefield for Israel and Palestinian guerrillas.

The diplomatic missions aimed at keeping Lebanon from exploding:

* Special US envoy Philip C. Habib, who was instrumental last year in averting a Syrian-Israeli war and in quelling a major Palestinian-Israeli blowup , was heading back to Lebanon Feb. 24. He expressed optimism about the situation to the Lebanese newspaper as-Safa prior to departing and predicted that Lebanon would not take a turn for the worse.

With Mr. Habib shuttling in and out of Lebanon, it is unlikely the Israelis will attack the Palestinian guerrillas, breaking a seven-month-old cease-fire (that Mr. Habib personally worked on), and further straining US-Israeli relations. However, Israel's new ambassador to the US, Moshe Arens, warned Feb. 24 his country would move militarily against the Palestinians if there are new attacks on Israeli citizens.

* The Arab League's follow-up committee, composed of the foreign ministers of Saudi Arabia, Kuwait, Syria, and Lebanese leaders, was due to meet in Beirut March 1. Its efforts last summer resulted in a multiparty agreement in Lebanon to keep the internal security situation from further deteriorationg. Since June 7, 1981, Lebanon has not had a major interparty conflict -- at least not openly.

* The March 2 visit to Israel by French President Francois Mitterrand, and Egyptian President Hosni Mubarak's trip to Israel the following week. The visits could help keep Israeli generals from moving into southern Lebanon. Both heads of state are attempting to develop balanced Middle East policies between Israel and the Palestine Liberation Organization.

In New York, meanwhile, the United Nations Feb. 24 considered a Lebanese request to increase the size of the UN peacekeeping force in southern Lebanon from 6,000 to 7,000 men. Most sides in the complex southern Lebanese difficulties support the increase.

The Lebanese government wants the area calm, as do the predominantly Shiite villagers. Israel wants the border further sealed off from Palestinian infiltration. And the PLO wants a bigger UN obstacle for the Israelis to deal with if they decide to move on the territory.

Besides the threat of an Israeli-Palestinian clash, the darkest shadow over this country today is being cast by Syria.

Having contained the uprising earlier this month in the town of Hama, the regime of Syrian President Hafez Assad is under pressure to pull out some, or all of its 22,000 to 30,000 troops in Lebanon.