First, you are misreading what your first source is. The document you reference is a UN interum report on Resolution 425. 425 dates back to 1978. That document (the report) does not reflect any agreement between the UN and Israel. 425 calls for many things, but there had been no public pledge from Israel on dismantling the SLA. In fact, Israel in public had been saying the opposite. They were not planning to leave until early July. The report being issued on the 22nd isn't very meaningful. Its coincidence. There was a whole series of meetings between the SLA (see later material) and UN/various countries that was a whole lot more damaging than the interium erport on 425.

Second, I think its better to deal with Charbel Barakat's fuller and more accurate version of what happened in May from his congressional testimony in June 2000:

"During the month of May 2000 our people remitted a memorandum to the office of the United Nations in Nakura in south Lebanon offering suggestions to avoid bloodshed and exodus. During the same month delegations representing our community met with diplomats of the US, UK, and French embassies in Israel and at the United Nations. And on May 18, our representatives met with the office of the Secretary General of the United Nations in New York ... Instead of taking our demands for protection on our land seriously, the United Nations declined to assist us ... The United States, UK and France dismissed our fears and insisted on guarantees which were allegedly granted by the Beirut regime ... The State of Israel only prepared for our potential exodus, ruling out our determination to remain on our land and defend ourselves."

"Despite the abandonment by the international community and against all odds, our people decided to remain and fight for their land and rights while calling endlessly on the UN to deploy its units around our villages. 
On May 22, 2000, and as Israeli units were implementing their own withdrawal and the commander of the SLA was absent in France, the mostly Shiite sector of the security zone collapsed. It appeared that Shiite officers were told by the Israelis that they must leave before the borders closed. Even after the invasion of Hizbollah of this area, the mostly Christian-Druse eastern enclave and the Christian western enclave, decided to defend themselves and protect their citizens after Israel’s withdrawal.
On May 23, 2000, at 8 PM, SLA officers in the western sector received orders from the Israelis to leave their posts as Hizbollah was advancing towards their villages. Thus the SLA was imploded from the inside and was dismantled without any battle. The main reason why the SLA did not remain and fight was the fact that the border was closed behind its back. Without access to medical and logistical supplies, our people was condemned either to surrender to its enemy or flee across the border." (from [www.10452lccc.com/special%20studies/barakatjune2000.htm])

The SLA did collapse. What Charbel Barakat is talking about in the MEF comments is that the remainder of the SLA (Christian) in the west did not collapse and was dismanted by Israel. But it was only dismanted after most of the SLA had collapsed. Charbel Barakat and other dead-enders may have wanted (in theory anyway) to fight on in the west, but Israel wasn't going to make any commitment to keep a miniture version of the SLA going in a tiny pocket on the west end of the border. The caution I would give about Barakat and a couple others from the SLA is that sometimes they think of their Christian portion of the SLA as if it were the entire SLA. And from that point of view, the SLA did not collapse and was dismantled by Israel.

The biggest claim made by Barakat is in weasel-words: "It appeared that Shiite officers were told by the Israelis that they must leave". 'It appeared that" means he is guessing. Nobody on either side of the border has outright claimed that the Shites were told to leave and that is why they collapsed on the 22nd. Even Barakat is unwilling to make that claim.

Now lots of other things may have happened that the public doesn't know about with regard to the SLA collapse. But the only people who would know the truth are a handful of people at the top of the Israeli government and army. Nobody in the SLA knows much of anything because if Israel was going to collapse them, they would hardly tell them their plan. People (including me) can make lots of hunches and theories about what might have happened, but there are no facts to back any of those theories up.