when John writes that he saw martyred souls alive in heaven
There is no verse that leads us to think these souls are in heaven. John was on earth when he saw the angel come down to the pit, that had been opened at the 5th Trumpet. Whether symbolic or not, still symbolically or not, on the earth.
These souls are the camp of the saints attacked later in the chapter. The thousand years is not a split vision, where one half of the thousand years is in heaven, and the second half on the earth. Satan was bound while on the earth, and loosed back to being on the earth. Those souls were on the earth, because they were just beheaded on the earth. You should at least state those souls were symbolically reigning in the camp of the saints off and on, to make any sense. The camp of the saints is not in heaven.
The 7th Trumpet is when Jesus is proclaimed king of every nation. Is that your last trumpet? If the redeemed are currently reigning on earth prior to the 7th Trumpet, why are we called ambassadors since the Cross? An ambassador represents a kingdom that is not presently ruling over the nation that one is an ambassador in. It always made sense to say in life, we are ambassadors, and in death we reign with Christ.
Why were these souls even judged? John mentions thrones set up and judgment given in relationship to these souls on the earth. I don't see how these souls were even made alive until they were judged via thrones set up on the earth. There is no implication they were set up in heaven. Heaven already has thrones since Genesis 1. The GWT exists after heaven and earth no longer exist, why would it not have been there before they existed? Why not have been judging souls since Adam and Eve?
The rest of the dead were not souls in heaven, so again those beheaded souls did not have to be either, the rest of the dead implies those souls were part of those dead, but were seperated and judged and then made alive, on the basis they had been beheaded. The rest of the dead stood in judgment before the GWT. Nothing states they immediately came to life at the split second the thousand years ended. The thousand year reign were for those souls judged and made alive. The point was that those other dead would have no judgment until after the thousand years. And then we see clearly they were judged at the GWT. It would be only after judgment some could be made alive, while the majority would be tossed into the LOF.
Most do not even seem to notice those thrones set up and judgment given. Those beheaded souls had to face the judgment, because they were not redeemed and judged with Christ on the Cross. This was not an awards judgment. This was receiving the first resurrection or being made alive status, which means the restoration of body and soul.
Obviously you do not include a physical body in the being made alive gift of God in Christ. You prefer sinful humans spiritually redeemed reigning over other sinful humans who have not accepted the gift of salvation. Except we see from history turning the church into ruling and reigning while in Adam's dead corruptible flesh does not always turn out that great. We have not been made alive is the point. We are still in Adam's dead state whether physically alive or physically dead. Because you don't even allow a physical body until the GWT judgment, which you call the last trumpet for some reason. A soul is always alive even in torment, and called the dead, until permanently placed in the second death, the LOF.
You still have to have souls continually standing in judgment before thrones during the whole thousand years. The first resurrection is the judgment awarded from these thrones.
"And I saw thrones, and they sat upon them, and judgment was given unto them."
This also has to be continuous if you have souls constantly dying and living throughout this time.