See the commentary by Martin V. Cisneros upon which this discussion is based:
Hosea 2:18: God's New Covenant between Man and Animal
Unfortunately, we could not contact Martin for you, so we will post your comments and question for him to see and reply to.
In the meantime, we’ll do our best to respond to your Ecclesiastes 3:21 question. In order to see the meaning of 3:21, we should look at verses 19-21, which states,
19 For the fate of the sons of men and the fate of beasts is the same. As one dies so dies the other; indeed, they all have the same breath and there is no advantage for man over beast, for all is vanity.
20 All go to the same place. All came from the dust and all return to the dust.
21 Who knows that the breath of man ascends upward and the breath of the
beast descends downward to the earth?
We see from verse 19 that the fate of both humans and other animals is the same, for both are spiritual beings with souls and spirits. In other words, spiritually, if humans go to heaven, then so do animals, but upon the earth both our bodies decay and once again return to the dust of the earth from which we came.
So, in verse 21, Solomon asks a question, which can only be answered in the negative, for no human actually knows for certain. We believe that he may have also been countering the people of his day, who believed that only human spirits rose to be with God, and the spirits of animals sunk down into Sheol (hell).
We hope this helps.
In the Love of the Lord,
Frank and Mary