Another viewpoint: The visions seen in each seal is not one of the events at all, because the events are written on the scroll, but each seal opens our understanding to the event written about on the scroll. For example,
"Voices, thunderings, Lightnings, and an earthquake" are always symbols of God speaking from heaven: We see it in Exodus 20:18, we see it used in poetic form by David in 2 Samuel 22:14-15, we see it proceeding from the throne of God in Revelation 4:5.
Only after reading the events written about in the seventh trumpet and the seventh bowl of wrath, do we realize that the vision John sees when the seventh seal is loosened, symbolizes God's judgment:
It's the 3rd "woe".
The reason the trumpets are introduced only when the 7th seal is loosened is because the seals
seal the events that are written on the scroll, and all the seals have to be loosened before the scroll can unroll, so once the 7th seal has been loosened, the scroll unrolls and we start reading about the trumpets.
But that doesn't mean the visions seen in the seals are not opening our understanding to the events we are told about in the trumpets and vials, because the same goes for 6 6 6: (6th seal, 6th trumpet and 6th bowl of wrath):
"..Fall on us and hide us from the face of Him sitting on the throne, and from the wrath of the Lamb;
17 for the great day of His wrath has come, and who will be able to stand?" (6:16b-17, 6th seal).
"Behold, I am coming as a thief. Blessed is the one who watches and keeps his garments, lest he walk naked and they see his shame." (Revelation 16:15, in-between the 6th and 7th bowl of wrath).
The second "woe" is linked to both:
(A) The sixth trumpet (Revelation 9:12-13); and
(B) To the period during which the two witnesses are prophesying (Revelation 11:3-14).
6th trumpet:
"And the number of the armies of the horsemen was two myriads of myriads. And I heard their number." (6th trumpet, 9:16).
6th vial:
15 Behold, I am coming as a thief. Blessed is the one who watches and keeps his garments, lest he walk naked and they see his shame.
16 And he gathered them into a place called in the Hebrew tongue Armageddon. (6th bowl of wrath, Revelation 16).
The 1st "woe" is linked to the
5th trumpet, which is the opening of the abyss, from which the beast ascends. He continues for 42 months and makes war against the saints and overcomes them (Revelation 13). He also makes war against the two witnesses who prophesy for 1,260 days, and when they finish their testimony, the beast kills them (Revelation 11).
And the
fifth angel poured out his vial on the throne of the beast, and
its kingdom became darkened. And they gnawed their tongues from the pain.
9 And when He had opened the
fifth seal, I saw under the altar the souls of those who had been slain for the word of God, and for the testimony which they held.
10 And they cried with a loud voice, saying, Until when, Master, holy and true, do You not judge and avenge our blood on those who dwell on the earth?
11 And white robes were given to each one of them. And it was said to them that they should rest yet for a little time,
until both their fellow servants and their brothers (those about to be killed as they were) should have their number made complete. (Revelation chapter 6).
I believe the visions seen in the seals open our understanding to what is written on the scroll, which only unrolls once all the seals have been loosened. Those visions are not events themselves. They are telling us something about the events. They are seals, so when they are loosened they open our understanding to the things written about in the scroll (which we only read when it unrolls, and it only unrolls when the 7th seal has been loosened).