There is much speculation regarding the answer to these questions.   

It is generally agreed that the one being restrained is Satan who will incarnate himself in the Beast - the man of sin.   

However, the ID of the restrainer is not agreed upon though the understanding of the past 200 years is that it is the church or the Holy Spirit.   

If we make the wrong assumptions about the restrained and the restrainer we draw incorrect conclusions.   

And now you know what withholds that he might be revealed in his time. 2Th 2:6 I rather think that `he which withholds` is Michael, the archangel Dan 10:13,20,21 Dan 12:1.      There are many who believe it is the church.      While yet others believe that it is the Holy Spirit.      The fact that there are Christians on the earth at the time of Jesus` return counters the supposition that the church is withholding.      The fact that the Holy Spirit is in the body of believers who are still on earth at the return counters the supposition that the Holy Spirit is withholding.

For the mystery of iniquity does already work: only he who now lets will let, until he be taken out of the way. 2Th 2:7 Satan is the mystery of iniquity.         → Who he was.         → Who he now is.         → Why he rebelled.         → What his plan is Michael (see comment on 2Th 2:6) is contending with Satan and keeping him from unleashing the fulness of his raw evil.      When Michael is taken out of the way (on direction from Jesus), Satan will incarnate himself in the beast, the Anti Christ.      At that point, he will manifest lying wonders with which to deceive the whole world.

And at that time shall Michael stand up, the great prince which stands for the children of your people: and there shall be a time of trouble, such as never was since there was a nation even to that same time: and at that time your people shall be delivered, every one that shall be found written in the book. Dan 12:1 This is the End Of The Age when the Beast arises creating the Great Tribulation.      What book?         → the Lambs Book Of Life.      Salvation is by Grace Through Faith In Christ Jesus as Mighty God, Saviour, Lord and King.      No Jews will be delivered from the Beast except by the blood of the Lamb and the Testimony Of Jesus Christ.

And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels, Rev 12:7 This is a very important verse to understand what is happening during the Great Tribulation.      There is war in heaven.         → There are the Lord's angels and    → there are Satan's angels.      In most passages throughout the Bible the term Angel Of The Lord or His Angel is used to make it clear whose person they represent.      In John's vision those terms are not used.     I suspect this is because He is not told who they represent - only what they do.     We can assume the angels are from the Lord, but that is likely not the case.      Jesus tells us that He will send His Angels to gather the wheat to Him Mat 24:31 and to gather the tares into bundles to be burned Mat 13:41.      He does not tell us of destructive angels which appear in the Revelation.     Since, He did not speak of such, I can only assume that the destructive angels in Revelation are working at the behest of Satan.,

And prevailed not; neither was their place found any more in heaven. Rev 12:8 The forces of the Lord headed up by Michael take all of Satan's turf leaving him to fall to earth with his horde of rebels.

And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceives the whole world: he was cast out into the earth, and his angels were cast out with him. Rev 12:9 The destruction of the Great Tribulation is a combination of the works of Satan's human servants and his angels that have been cast out of heaven.

And I heard a loud voice saying in heaven, Now is come salvation, and strength, and the kingdom of our Lord, and the power of his Christ: for the accuser of our brothers is cast down, which accused them before our Lord day and night. Rev 12:10 There is no more accuser before the Lord.     It is now a simple matter of Satan deceiving as many as he can before the end.

And they overcame him by the blood of the Lamb, and by the word of their testimony; and they loved not their lives unto the death. Rev 12:11 This is one of the most important verses in the book of revelation for each of us personally.      We overcome by two actions and one attitude.     The two actions    → Jesus shed blood    → Our testimony that Jesus is our Creator and Saviour.      The one attitude is    → Being willing to die for Christ`s sake.      Take up Your Cross and enter into fellowship with Jesus, experiencing His suffering and His joy.      The reason that we must love not our lives is that Satan will use all his power to turn our hearts from the Lord.     We must be willing to die for His testimony.

Therefore rejoice, you heavens, and you that dwell in them.     Woe to the inhabiters of the earth and of the sea!     for the devil is come down unto you, having great wrath, because he knows that he has but a short time. Rev 12:12 Satan has no spiritual power.     His only power is to lie and deceive.      But though he is reduced, his anger and wiles make a formidable front against humanity.      So, the destruction of earth during the tribulation is due to the arrival of Satan and his angels for the purpose of destroying humanity.      Unfortunately, many humans will be aligned with Satan under the leadership of the Beast to facilitate their own destruction.