There is no reason this symphony needs to be associated with one specific event or person. By this point in his life, Mahler was faced with loss on many fronts. Other than the loss of his daughter, he had experienced the loss of professional achievement (having departed the Vienna Opera and Europe, only to be replaced in his position at Metropolitan Opera by Toscanini), he had lost his dignity as his wife engaged in her affair with Gropius, he had lost his physical health (diagnosed with heart disease). In other words, by this point in his life, he was confronting mortality and loss and decline in the most important aspects of his life. This was sure to have been very demoralizing and depressing for him.
I believe that this symphony became his vehicle of catharsis, his way of releasing the emotions that dominated his life at that time, his "diary" of this passage through mortality. As such, I believe this symphony expresses the struggle every human eventually goes through when confronted with the realities of loss and death. Kubler-Ross identified five stages -- denial, anger, bargaining, depression and finally the acceptance of "fate". I believe Mahler was passing through those stages and that this symphony provides a "diary" of his experience. It was, in my mind, a farewell, but not in the way Bernstein described it. It was more personal than that.
Of course, years ago, when I first learned of the Kubler-Ross stages, they were just like any other theory. They had no personal meaning to me. As I have gotten older and begun losing friends and relatives and parents to death, this is no longer theory for me. Working in health care, I see this every day. The passages of life. All who live into adulthood eventually go through these passages of life.
As the great medical doctor and prolific author Isaac Asimov wrote: "Life is pleasant. Death is peaceful. It's the transition that's troublesome." I believe that Mahler documented that transition in his 9th.