@ Duran
Good question!
Why will the two witnesses not preach until after the 7th trumpet? I think the answer is in Rev 11:7: they are to preach and the Beast of Abyss will end their preaching. According to Rev. 17:8, the Beast comes out of the abyss and is, I believe, identical to the Beast of Rev. 13:1 (the reason for the identical beast - aside from "cosmetic" differences in description - is that this Beast comes out whole with its heads and also goes whole into the "lake of fire" in Rev. 19:20).
This means that the two witnesses have to preach for 1260 days, and sometimes at the end of their preaching ("they finish preaching" Rev. 11:7) the Beast appears for 42 months, which at the beginning of the reign, destroys the two witnesses.
This is probably obvious to you, but like you, I have addressed the question, don't the 1260 days begin at the time of the 6th trumpet (2nd woe)?
The decision for the 1260 days only beginning at the time of the 7th trumpet, in my opinion, lies in the use of the future tense for the verbs "prophesy" or "trample". Therefore, I have placed the beginning of "prophesying" only in the 7th trumpet.
I explain it another way: in the broader context of the chronology of Revelation as I understand it, if I add up the 1260 days (sermons) and 42 months, I come up with a length of these events of about 7 years. Okay, but what about the preceding period? As has been discussed here, I consider Joseph's dream in Egypt to be "eschatological": Ex 41:49 describes that in the first seven years, the "corn" (= sons of God) was like "sand in the sea" that they stopped counting them. In Ex. 41:50 "before the year of famine began", two sons (=two witnesses) are born to Joseph. Then sets 7 years of famine...
The total time from the hanging of the four (angelic) horsemen to the destruction of the Beast would be about 14 years. Which also corresponds to the requirement, so well known in JWs-theology, that the generation "shall not pass away". According to Matt. 10:21, the father is to deliver the child and the children, his parents, to death. It does not say (though it probably does!) that grandfathers their grandchildren or that grandchildren their grandfathers.
The 14 years or 7 years for the harvest* also suggests the duration of the individual breaking of the seal and trumpet: in the 5th trumpet the torment by the locusts is described (Rev 9:5;10) - it is to last 5 months. I infer from this that it is roughly a total of 6 months for the individual seals and trumpets. If I add these six months together from the first seal to the 7th trumpet, it is the 7 years. Some clue to the "half-year" duration is found, I believe, in the text of Rev. 9:15, where the angel is prepared for the hour, day, month, and year. The event is to take place within a year, not e.g. seven years or 50 years (Jubilee).
* The cleansing and gathering of the last of the harvest (144,000) and the gathering of all others who show the slightest humanity (like the thief on the cross) or just obey(!), like the 276 people who were with Paul in the boat (Acts 27:37), then likewise from the world and from the power of the Beast, all people will be saved. All the saved, including the 144,000, will be as the sand in the sea, these are the ones coming out of the great tribulation... if the end comes anytime soon, then I think billions will survive...