Conclusion

In this third major division, John has done the same thing. He will do it again in the fifth major division.
Another "confusion" is that in the fourth major division, John is going to show us Judgment Day itself -- and then is going to show us Judgment Day itself in the 6th major division. Can't John get a grip on himself and speak in an orderly manner?
John is speaking in an orderly manner, but he is showing the Church a picture of the events of the End as seen from several perspectives: Their own, The pagans, God's.
The End of History:
II. The 7 Seals = the tribulation seen from God's point of view.
III. The 7 Trumpets = the tribulation as it will appear to the martyrs (and also to the Church at large).
V: The 7 Plagues = the tribulation as it will appear to the godless world.
The Judgement of all people:
IV: The Salvation of the Church - Last Judgment as seen by Christians.
VI: The judgment of the world - Last Judgment as seen by Pagans.
VII: The Church in the Millennium: The Final Triumph = a view of the new age from the Church and God's point of view.
Consequently, the elders sing in the past tense because at this point, the end has come. It will look different from this in division V, because division V will show how the End will appear from the perspective of the pagans.
This glorious hymn is called the "third woe" by John. How can a woe be so glorious? Because in Rev. 11:15-18 you are seeing the "third woe" from the perspective of the resurrected Christians.
You want to see how this third woe will look from the perspective of those Godless ones still living. Turn to Rev. 17:1 - 20:3 (Division V) but don't do it just yet: we have to go through Division IV first and see how the Final Judgment looks to the Church.
Conclusion to Division III, The 7 Trumpets:
In this vision of the 7 trumpets we are seeing the last days of earth as we know it from the viewpoint of Christians on earth