Hello Diego, Welcome to PB!
You said,
Question 1: If Satan is bound (Revelation 20) then how is he still affecting the Church (1 Thessalonians 2:18)?
#1, In 1 Thess 2:18, Satan is
allowed by the Sovereign LORD to prevent Paul from going back to the Thessalonians, as He has work for him to do in Athens. So Paul sent Timothy instead. Here Satan is but an instrument for effecting the will of God.
Question 2a: Does the Thousand Years represent the entire New Testament period as a whole?
#2a, No.
A top Amil commentator, Dennis Johnson, puts it like this, "Amillennialists . . . see the thousand years as symbolizing the age from the exaltation of Jesus
until just before his second coming. The binding of the dragon means that throughout this period,
until just before the end, Satan cannot hold the nations in darkness, blinded to the gospel; specifically, he cannot assemble an organized, worldwide conspiracy to attack the church through violent persecution." [Emphases added] (Dennis E. Johnson,
Triumph of the Lamb: A Commentary on Revelation). After the "1,000 years" of Satan's being bound, when those thousand years are expired, Satan shall be loosed out of his prison to gather the nations to attack the church.
Question 2b (if A was 'yes'): In Revelation 20, the thousand years end before Christ's second coming. How can the millennium represent the entire period?
#2b, Although answered in my #2a, let me let Johnson speak more on this: "What is the historical-chronological relationship between the physical second coming of Christ and the age designated the thousand years in John's vision? . . . In the context of the drama of Revelation, this purpose statement makes clear that the historical-chronological referent of 'the thousand years' in which the dragon is 'bound' must precede the battle that John just saw, in which the rider vanquished the beast, the false prophet, and all their followers (Rev 19:11-21).
The millennium must occur before the second coming and the last battle for several reasons. . . . [emphasis added]
"Although it is true throughout history that Satan, the ancient serpent, "deceives the whole world" (Rev. 12:9), in this vision a specific deception to obtain a specific objective is in view. We see this objective when,
at the end of the thousand years, the dragon is released and comes out 'to deceive the nations which are in the four corners of the earth, Gog and Magog, to gather them together for the war' (20:8). 'The war,' as we have seen, is a reference to 'the war of the great day of God, the Almighty,' for which the world's kings were gathered by three unclean spirits that proceed from the mouths of the dragon, the beast, and the false prophet (16:13-16)." [emphasis added] (Dennis E. Johnson.
Triumph of the Lamb.)
I think this is pretty clear. "The war" spoken of is Armageddon, when the gathered nations of the world attack the camp of the saints — the
global church of Christ — so as to destroy them. It is in the midst of this attempt the Lord returns to save His people and execute vengeance on the marauders.
Question 3: Satan's bondage will be reversed for a time; therefore what was done to bond Satan is reversible. It cannot be Christ's accomplishment on the cross because that is irreversible. So then what was it that bonded Satan?
#3, Satan's "bondage" as you put it, that is, his chaining, pertains to being unable to gather the nations,
as nations on a global basis, to attack and destroy the saints. This binding of Satan limits his influence
to individuals within the nations of the world — he is certainly able to deceive
them — but the unbinding of Satan allows him to deceive entire nations.
The quote above of Warfield, "The saints described are free from all access of Satan - he is bound with respect to them: outside of their charmed circle his horrid work goes on" is
wanting. Satan can indeed attack the saints both while and after his binding — to wit, he can kill them, cause them suffering of various sorts. We are
not "free from all access of Satan" — he is allowed to persecute us in the furnace of affliction. But we "overcome him by the blood of the Lamb, and by the word of our testimony; and we love not our lives even unto the death." (Rev 12:11)
I hope this answers your questions, Diego.