If this whole Olivet Discourse is one prophecy toward one event (70 AD), then how does this fit in?

It says, "When the Son of Man comes in his glory." But we already read in 24:30, "Then will appear in heaven the sign of the Son of Man, and then all the tribes of the earth will mourn, and they will see the Son of Man coming on the clouds of heaven with power and great glory." This was followed in 24:34 with, "Truly, I say to you, this generation will not pass away until all these things take place."

Also, in Matthew 16:27, 28, Jesus said, "For the Son of Man is going to come with his angels in the glory of his Father, and then he will repay each person according to what he has done. Truly, I say to you, there are some standing here who will not taste death until they see the Son of Man coming in his kingdom.”

These both describe the coming of the Son of Man, in glory, with angels, as a King, to judge, universally ("all the tribes of the earth", "each person"). And in 16:28 during some of the disciples' lifetime.

So it is very easy and natural to see these passages talk somehow about the same event.

Matthew 25:32, however, says, "Before him will be gathered all the nations." Were all the nations involved in the fall of Jerusalem?

Let's look at universal language in the Bible.

Matthew 24:22, in speaking of the tribulation said, "And if those days had not been cut short, no human being would be saved." Yet this tribulation was period before the destruction of Jerusalem and only involved the area of Judea.

Could "all the nations" (25:32) be the same as "all the tribes of the earth" (24:30). They do seem to be parallel.

In Genesis 17, God promises Abraham that he has "made you the father of a multitude of nations." Romans 4:17, 18: "As it is written, 'I have made you the father of many nations'—in the presence of the God in whom he believed, who gives life to the dead and calls into existence the things that do not exist. In hope he believed against hope, that he should become the father of many nations, as he had been told, 'So shall your offspring be.'"

The word nations here is ethnos from which we get our word ethnic. We also get the word ethnarch, which is "the ruler of a province or people" (American Heritage Dictionary). The word ethnos is the same word Josephus used to reference the "nation of the Samaritans," "the nation of the Galileans," etc. Each of these places at times were ruled by ethnarchs. So it is well within the common usage of the time to understand the nations as the "nations" of Palestine.

Notice this is the same word used in Matthew 28:19, "Go therefore and make disciples of all nations." If the apostles had understood this "correctly" as evangelizing the entire globe, then why were they so surprised in Acts 11:18, "When they heard these things they fell silent. And they glorified God, saying, 'Then to the Gentiles also God has granted repentance that leads to life.'" If Peter had understood this globally, why did he not see his call to the uncircumcised as a fulfillment of the commission of Matthew 28?

This lets Matthew 10:23 make more sense, "When they persecute you in one town, flee to the next, for truly, I say to you, you will not have gone through all the towns of Israel before the Son of Man comes."