U.S. officials, frustrated by the impasse on withdrawal of Israeli forces from Lebanon, hope that Israel's inquiry into the massacre of Palestinian civilians in Beirut last September will help break the deadlock and lead to progress on President Reagan's broader Mideast peace initiative.

The findings of the independent commission investigating the circumstances of the massacre are expected to be made public in about two weeks, and administration policy-makers feel that by the end of the month, the United States could be facing a very different situation in dealings with Israel.

In particular, U.S. officials are watching closely for the impact of the commission's findings on Israeli Prime Minister Menachem Begin, who has yet to give a clear signal of his position on the Lebanon question, and on Defense Minister Ariel Sharon, regarded here as the main impediment in efforts to modify Israeli demands on Lebanon that the United States considers unreasonable.

But, while the impending commission report is expected to produce dramatic political reactions within Israel, no one knows whether they will help to steer the Lebanon talks in the direction sought by the United States or make achievement of U.S. goals even more elusive.

In fact, U.S. officials, working on the assumption that the commission will find some degree of negligence and culpability by the Begin government and Israeli military, foresee four possible scenarios that could emerge in the aftermath of the report:

* The findings could be so damaging and condemnatory of the government that the Begin's coalition junior partners, the national religious parties, might split with his Likud bloc and bring down the government. That could lead to formation of a new government joining the religious parties with the opposition Labor Party, whose mainstream is regarded as more flexible in dealing with the United States.

* Begin could use the commission report as a reason to set elections that would be a plebiscite on Israeli policies in Lebanon and his hard-line resistance to the peace initiative proposed by Reagan last Sept. 1.

That would mean a caretaker government in Israel for several months, during which any hopes for progress on Arab-Israeli issues would be put on hold.

But in the end, there would probably be a clearer picture of how far the Israeli electorate is willing to go in straining U.S. ties.

* If the commission's criticisms fall most heavily on Sharon and the military, Begin might try to force him out.

Sharon's departure from the Cabinet would mean an increase in the influence of Foreign Minister Yitzhak Shamir and others who, while relative hard-liners like Begin, are regarded as far more sensitive than Sharon to the need for finding an accommodation with Washington.

* Sharon, seeking to block attempts to dump him, might try to force the government to make some new move in Lebanon that would drastically change the equation there and open the way for a new U.S.-Israeli confrontation.

The most likely maneuver of this sort would be a unilateral Israeli pullback to a security zone extending about 45 kilometers north of the Israeli border.

That would leave a military vacuum in the southern half of Lebanon at a time when the Lebanese army is unable to assert control there. Instead, the void would probably be filled by Lebanese Christian irregular forces aligned with Israel.

If they were to succeed, the U.S.-backed government of President Amin Gemayal would be totally undermined, turning southern Lebanon into a rump state under Israel's thumb and ceding the northern half of the country to Syria since Syrian forces and their Palestine Liberation Organization allies in the north would not withdraw from Lebanon if Israel dominates the south.

The prominence given to Sharon in these scenarios underscores the degree to which U.S. policy-makers regard his political future as the key to whether quick progress can be made toward resolution of the Lebanese situation.

According to U.S. officials, the real problem in the Lebanon negotiations has not been the haggling about specific issues such as security arrangements for Israel's northern borders or Lebanese recognition of the Jewish state. It is the fact that the United States has found itself contending with two distinct Israeli power centers.

In the U.S. view, one group, centered on Shamir and foreign ministry professionals, consists of relative moderates who, while seeking to win maximum concessions for Israel, believe that an accommodation must be reached with the United States.

The other, led by Sharon, is regarded here as maneuvering to cut the United States out of a major role in Lebanon and bring the country under long-term Israeli domination.

It is an open secret that U.S. officials regard Sharon as responsible for the series of incidents in Lebanon, including last week's confrontation involving U.S. Marines and Israeli tanks. These officials see a deliberate attempt to discredit the multinational peace-keeping force in Lebanon and prevent it from expanding its functions to fill a military void if Israelis and Syrians withdraw.

Complicating the situation has been what U.S. officials see as the uncharacteristically enigmatic posture of Begin, the only man with sufficient strength to rein in Sharon. The officials say they do not know whether Begin has not recovered from the emotional shock of his wife's recent death or whether he simply is biding his time.

He has stayed unusually aloof from the Lebanon talks, and the hope here is that release of the inquiry findings will force him to make clear how far he is willing to go in bucking the United States over Lebanon.

Dependent on the answer are such top-priority policy goals as eliminating Lebanon as a source of Middle East tension and then moving on to Reagan's broader goal of inducing King Hussein of Jordan to join expanded negotiations on the future of Israeli-occupied Arab territories.

Administration officials concede that the United States is far behind its original timetable for withdrawal of foreign forces from Lebanon by last December and faces a setback to its hopes of drawing Hussein into the process if it cannot demonstrate to him U.S. effectiveness in the Lebanon situation.

Yet, U.S. officials seem unanimous in agreeing that, while the Reagan initiative clearly has become a hostage to the Lebanon problem, the stalemate cannot be broken by threats to cut off aid to Israel or subjecting that nation to other pressures.

In fact, some officials argue that such talk is counterproductive because it only serves to deflect the Israeli public's attention from the impending commission report.

Even the expected return to the Middle East this weekend of Reagan's special envoy, Philip C. Habib, is regarded here as basically a holding operation that, at best, might succeed in narrowing a few of the differences in the bargaining over Lebanon.

At the same time, the officials acknowledge that, if Reagan's policies are to be kept from bogging down, the administration cannot passively allow another six months to slip past.

At various levels of the administration, various options have been discussed, including a possible trip to the region by Secretary of State George P. Shultz, a carrot-and-stick strategy about how much of the bill the United States might foot for Israel's costly incursion into Lebanon and even the idea of an eventual crackdown on military aid.

But, the officials caution, all such ideas are in the most preliminary discussion stage and unlikely to be pursued further for some time, if at all.

For the moment, they say, the United States has no choice other than to wait for the catalytic effects of the commission report to produce reactions in Israel and then fashion a strategy for dealing with whatever internal Israeli political situation emerges from the fallout.