Ecclesiastes is not an eschatological book, but a philosophical one. I do not believe all books of the Bible to have equal weight and this book's outlook on human existence is dreadfully cyncial and supercilious. If one wishes to know what happens after death, I recommend the New Testament view that the spirit puts off the body as it does a garment. As Jesus' body lay in the tomb, his spirit went to preach to the "spirits in prison," which were disobedient in the days of Noah. He did not go to heaven, and neither did the thief on the cross. Jesus also taught the parable of Lazarus and the rich man. Ah, but that's just a parable, some may say, and yet when did Jesus ever give a parable that wasn't true to life?
When the apostles asked Jesus, "Lord, who did sin, this man or his parents that he was born blind," can anyone miss the implication? The apostles assumed not only that the spirit had existence beyond the grave, but before birth as well. In everything that has been learned from ancient texts discovered in the last one hundred years, none proceeds from the premise that man does not have an immortal spirit.
Without the Ecclesiastes reference, the notion that man doesn't have a spirit pretty much falls on its face. The writer believes that everything is futile that man does, a notion that is not taught anywhere else in scripture. Jesus never refers to it in his sayings as he did other prophets, like Isaiah.
As far as the words for "breath" and "spirit" are concerned, yes, they can be used interchangably in either Hebrew or Greek. The trick, then, is to guess correctly. Most Christian scholars have said, yes, man does seem to have an immortal spirit. I not only believe man has a spirit, but that animals also possess spirits. Paul writes: "The Spirit itself beareth witness with our spirits, that we are the children of God." (Romans 8:16) Here and elsewhere, it's implied that spirit not only has intelligence but gleans intelligence from the Spirit of God.
I don't find a great deal of wisdom in Ecclesiastes; rather, it seems to have been written by someone who has been beaten up by life and has made plenty of wrong decisions along the way. He speaks relentlessly on the futility and the meaninglessness of life; that it's fleeting and transitory, and then it's over. The Gospel of Christ should dispel such notions, for it proclaims that life can be rich and meaningful.
It remains a topic of debate, but that's my take.