Acts 3:21 "Whom the heaven must receive unto the times of restitution of all things, which God hath spoken by the mouth of all His holy prophets since the world began
66 AD is not the restitution of all things.
That verse seems to me to be rather meaningless by itself since the Greek word ἀποκαταστάσεως is used but once in the NT and LXX; but it comes to life when interpreted in the context of Peter's address, beginning at verse 12. Peter is speaking to the Israelites to confirm the covenant God made with them (vs. 25), which is conditional on their repentance (vs. 19). Those who would not repent would be cut off (vs. 22-23); those who repented would be blessed (vs. 26). Peter's sense of urgency implies he was speaking of things that they, themselves, would be experiencing, not remote, future generations.
The root word, ἀποκαταστ, is found but three times in the NT, twice translated to "restore" (Matt 17:11, Heb 13:19.) Therefore my best guess is verses 19 and 21 refer to the restoration of the two houses into one in the heavenly kingdom, as foretold by the prophets and as expected by Paul:
"And the Lord shall deliver me from every evil work, and will preserve me unto his heavenly kingdom" -- 2Tim 4:18 KJV
Acts 1:11 "Which also said, "Ye men of Galilee, why stand ye gazing up into heaven? this same Jesus, Which is taken up from you into heaven, shall so come in like manner as ye have seen Him go into heaven."
He will come and stand upon the mount of Olives where he will cleave a path -
That verse doesn't say that Christ would stand again on the mount of Olives, only that he would come from heaven as he went into heaven, e.g., in a cloud (Acts 1:9).
Zechariah 14:4 "And his feet shall stand in that day upon the mount of Olives, which is before Jerusalem on the east, and the mount of Olives shall cleave in the midst thereof toward the east and toward the west, and there shall be a very great valley; and half of the mountain shall remove toward the north, and half of it toward the south."
The theme of Zechariah 11-14 includes the betrayal of Judas (vs. 11:12-13), the crucifixion (vs. 12:10), the scattering of the disciples (vs. 13:7), the cutting off of the unrepentant (vs. 13:8), the refining fire to try the faithful (vs. 13:9), and the destruction of Jerusalem (vs. 14:1-2).
I will not pretend to understand the passages that the prophets did not understand, nor were explained by Christ or the Apostles. You can find many different interpretations of Zechariah 14:4-5 in the commentaries. My interpretation is that no one alive today has a clue what that passage means.
Matthew 24:22 "And except those days should be shortened, there should no flesh be saved: but for the elect's sake those days shall be shortened."
It would be impossible for this to be the greatest tribulation of all time. We've had world wars worse than this.
Matthew 24:21 "For then shall be great tribulation, such as was not since the beginning of of the world to this time, no, nor ever shall be."
Future
The great tribulation occurred while some of Christ's disciples were still alive (Matthew 24:34.) Those who have not studied the history of those times are probably not aware of the suffering that the elect experienced under Nero beginning in 64 AD, and that the Jews suffered under the empire-wide persecution and murder by the Romans after the rebellion began in 66 AD.
Further, if any of the things mentioned in the Olivet Discourse were fulfilled during that generation, then all were fulfilled. For example, we are fresh out of disciples:
"Then shall they deliver you [disciples] up to be afflicted, and shall kill you: and ye shall be hated of all nations for my name's sake." -- Mat 24:9 KJV
And we are fresh out of Judaeas:
"And when ye shall see Jerusalem compassed with armies, then know that the desolation thereof is nigh. Then let them which are in Judaea flee to the mountains" -- Luk 21:20-21 KJV
Two years into the Neronic persecution of Christians, they were delivered:
"And he shall send his angels with a great sound of a trumpet, and they shall gather together his elect from the four winds, from one end of heaven to the other." -- Mat 24:31 KJV
Plus Paul is teaching about his return to non Jews and teaches the Esphesians that they need to have the gospel armor on to stand in that "evil day". They're not in Jerusalem. The event in 1st Thes 4 is the same event in Matthew 24.
The persecution of Christians by Nero was empire-wide.
And the biggest thing is Satan and his have not arrived yet. That's prophesized in Revelation 12. And they overcome him by the blood of the lamb and "the word of their testimony".
The scripture states that Satan was kicked out of Heaven (Rev 12:9), and for a short time he made war with those who had the testimony of Jesus (Rev 12:12, 17). After that short time, Satan was bound for "a thousand years", which is a long time, not one thousand years.
I believe that Satan was bound when Jerusalem and the Temple were destroyed. I also believe that he has now been released from his prison and he and his armies are currently deceiving an unsuspecting world (Rev 20:7-8). His primary target is the Church (Rev 20:9). He will fail.
The disciples asks for signs to the end of the world/age and his return so anything he tells them would be future
Yes, everything he told his disciples was to occur in their future, but within their generation. Some of the signs that we can easily understand were the falling away of Christians (mentioned throughout the NT); the persecution of the disciples; Jerusalem being surrounded by armies; and even the Gospel being preached to the whole world, to all nations, and to every creature (Paul wrote that all three were fulfilled during his ministry).