Rex, I doubt if the numbers we're talking about would have tipped the balance in Jordan. Hussein was willing to use any amount of force, and his Jordanian arab tankers of the 40th and other divisions proved themselves politically reliable enough to do the job, regardless of the unpleasantness of the fight. Amman was a tough nut to crack, but it cracked in the end just the same. An army with the political will to use overwhelming force will win a kinetic fight in urban operations.
Actually, it was a close run thing. Despite post-conflict Jordanian military myth-making, the Jordanian 40th Armoured Brigade did quite poorly: when the Syrians/PLA blundered into the Jordanian formations, the latter showed little tactical skill and the Syrian armour eventually inflicted heavier casualties and pushed the Jordanians off the ridges at al-Ramtha.

By the end of the battle on 21 September 1970, Husayn feared that he had lost the war. The Syrians looked like they would break through to Amman, where the Jordanian 4th Mechanized had made only limited headway against the PLO (indeed, much of the capital was still in PLO hands at this time).

The next day was critical: the Jordanians launched massive air attacks against the Syrian/PLA troops. Syrian DM Hafiz al-Assad feared escalation, and refused to commit the much larger Syrian AF, despite orders to do so. It was over the next two days that Syrian intervention was defeated, and the Syrian/PLA troops withdrew. Without Syrian/PLA support, the PLO would eventually lose too, although it wasn't until April 1971 that they lost control of the last Jordanian towns, and they weren't fully defeated until July.

Would al-Assad been able to do this if the Israelis had done a Kosovo or 1948-style ethnic cleansing of the West Bank? I'm doubtful, given political dynamics in the Syrian Ba'th Party at the time.

Moreover, ethnic cleansing would have added over half a million additional bitter refugees to the PLO's potential recruit base, and probably further radicalized Palestinians in the Army (several thousand of which defected in any case).

Had the SAF been committed against the RJAF, Israeli and/or US intervention would likely have followed. The IAF could have taken on the Syrians, but at the cost of further delegitimizing King Husayn. I'm doubtful the regime would have survived.

We'll never know though, will we?