| Quote #4
I saw, before I had well finished,
The speaker's inability to finish what he is doing, and the swans' sudden departure ("suddenly mount," "scatter"), point to the suddenness of death and the abandonment that goes with it.
| Quote #5
All's changed since I, hearing at twilight,
The speaker used to tread "with a lighter tread," but doesn't anymore. That carefree part of his life is over, dead and gone. It is not totally ridiculous to think of life as a series of deaths—things change ("all's changed"), people enter and leave our lives, etc.
| Quote #6
[…] when I awake some day
The swans' disappearance is a kind of death. In a way, the speaker imagines the swans leaving him because it is easier to accept their "death" than to think about the undeniable truth that he, too, (and other people he knows) will die someday.