Hi on the subject of afterlife, there are at least three perspectives that biblical authors can use, view in heaven, view in sheol, and view on earth. Both Psalms 6 and Ecc 9 are speaking from an earthly viewpoint. A person on earth can not see what is happening in heaven or happening in sheol. Ecc 9, Solomon is in a state of apostasy, and is speaking as a man on earth, he does not see what people in the grave are or are not doing. He is speaks as one, “under the sun”, he says “a live dog is better than a dead lion”. Read Psalm 6 for context, David is in dispair, and he is speaking from a earthly viewpoint. From his perspective on earth those in sheol are not doing anything. Now, look at Isaiah 14, it speaks from the viewpoint of those in sheol, they rise up to greet the new person who enters the world of the dead, they speak, they greet. Now, look at Rev 6 ( it is future ), but it reflects what is happening in heaven, John is viewing this in heaven itself.
Here is another example of perspective that has nothing to do with your text,
1 John 4:12-21 (King James Version)
12No man hath seen God at any time. If we love one another, God dwelleth in us, and his love is perfected in us.
13Hereby know we that we dwell in him, and he in us, because he hath given us of his Spirit.
14And we have seen and do testify that the Father sent the Son to be the Saviour of the world.
15Whosoever shall confess that Jesus is the Son of God, God dwelleth in him, and he in God.
16And we have known and believed the love that God hath to us. God is love; and he that dwelleth in love dwelleth in God, and God in him.
17Herein is our love made perfect, that we may have boldness in the day of judgment: because as he is, so are we in this world.
18There is no fear in love; but perfect love casteth out fear: because fear hath torment. He that feareth is not made perfect in love.
19We love him, because he first loved us.
20If a man say, I love God, and hateth his brother, he is a liar: **for he that loveth not his brother whom he hath seen, how can he love God whom he hath not seen? **
21And this commandment have we from him, That he who loveth God love his brother also.
Basically, John is saying that people normally in everyday life do not see God and if we don’t love our neighbor whom we can see, odds are we really don’t love God either. If you read I John 4:12 alone without paying attention to context, then the verse isolated can be used as an argument aganist the deity of Christ. But, once you read the text, and realise that John is speaking in general terms of not seeing God, then you realise that this text does not rule out all the theophanies of the OT.