OK - remember this is 1973.
The PLO has already been around for 10 years. The PLO, which had at least lip service support from ALL the Arab countries in 1973 had a policy of the complete elimination of the State of Israel. This means that these states, at least as far as what they said, agreed with this goal. Of course in their heart of hearts they may not have agreed, but nobody can speak to that. Based on what the various Arab states, to say nothing of the PLO and other Palestinian groups said OPENLY the result of a complete victory would be the elimination of the State of Israel and the removal of the Jews who had not been there during the Mandate or even sooner (depends on which speech you listened to). Basically everything the Jews could not carry with them; businesses, apartments, autos, etc would stay behind without compensation - and it is questionable whether things like money or jewelry would be able to be taken away.
Between 1948 and 1973 in various places in the Arab world you had the Jews of Iraq, a community that had existed there since before the birth of Muhammed, expelled with 2 suitcases and stripped of all valuables, the Jews of Yemen (an impoverished group) were expelled with what little they had confiscated, and other similar instances.
Based on what the Arabs, both "resistance groups" and governments were saying on a regular basis since 1948, and on the actions in many parts of the Arab world, to say nothing of the preachings of al-Husseini the Grand Mufti of Jerusalem, the Jews of Israel would expect the best case scenario to be a low level of rapes and murders during the fighting, followed by the bulk of them being expelled with little more than the clothes on their back. Worst case scenario, lots of dead Jews and very few living to be expelled. A lesson the world learned with Hitler was that when somebody tells you what they are going to do, believe them (and BTW Pol Pot and his merry men made no secret of their plans as another example). For the Israelis to think, "oh these threats from the Arabs are just hot air" would be suicidal.
As far as Egypt goes, you do not need to nuke Cairo etc - simply take out the Aswan High Dam and the resulting flood will completely take out those two cities and the bulk of Egypt's industry and population - just one low yield weapon is all it takes. As far as Russia goes, I very much doubt they would nuke Israel if as a desperation measure Israel unleashes nukes. Doing that has too many risks for Russia. In 1973 the Israelis had both the A-$ and F-4, both of which were certainly nuclear capable although not delivered as nuke versions could have been made so very easily. They also had air-to-air refueling capability. I am fairly sure that had the Israelis felt the Russians might play they had plans to have assets ready to strike Russia - low level one way missions at least as far as the Crimea if not further in Southern Russia were possible. Sure the Russians could have turned Israel in to green sand, but if they thought it would cost them a few cities in the Motherland - to retaliate for a few Arabs - get serious.
If the Israelis are losing that badly, unless the US Marines are landing and US aircraft are helping to drine the Arabs back across the borders (which ain't happening minus Skippy the ASB), the Israelis are not going to care about "US support".