Does this scripture depict that those "brethren", dead in flesh, are reining as Kings and Priests ?

Very GOOD question Eagle !!! Halleluiah !!!

This shows the intervention of the saints that has went before us ! Halleluiah ! " Under the Alter " !

Revelation 6:9,10

We SHALL reign as Kings and Priests, EVEN HERE IF WE HAVE THE FAITH !!!

I think you will find that it is the Angle with the a vile referred to in Revelation 17:1

Rev 17:1

And there came one of the seven angels which had the seven vials, and talked with me, saying unto me, Come hither; I will shew unto thee the judgment of the great whore that sitteth upon many waters:

That is being referred to in this verse -

Rev 19:10

And I fell at his feet to worship him. And he said unto me, See thou do it not: I am thy fellowservant, and of thy brethren that have the testimony of Jesus: worship God: for the testimony of Jesus is the spirit of prophecy.

All the angel says is that it is God who should be worship and not himself.

May be the GNB put it more clearly –

Rev 19:10

I fell down at his feet to worship him, but he said to me, "Don't do it! I am a servant together with you and with other believers, all those who hold to the truth that Jesus revealed. Worship God!" For the truth that Jesus revealed is what inspires the prophets.

I’m afraid I don’t see were the idea “the dead in the flesh” comes from. It is not an expression found in the Bible.

Read 1 Peter 2:9

I think that the person speaking was a Saint in heaven in his soul & spirit (no body). (Our new spirit blends with our soul when born again.)

When the marriage supper of the Lamb does happen, this will be when the 'caught up' to be with the Lord happens & people are metomorphed into their everlasting body. So they won't be dead in the flesh anymore, but in their new everlasting body.