[…] and if the friend who was with him as he was dying had not had an elder brother who also died quite young differently in peacetime leaving two children one of them with bad health (5-8)
This super-long quote is an example of how the speaker ties his existence to events and people that seem completely unrelated to him. Our existence, Merwin suggests, hinges on so much more than what is immediately apparent to us. Far out.
I would not have found myself on an iron cot (19)
The speaker is attempting to collect all of the random events that put him where he is today. It's an interesting point of view—he's trying to see himself in relationship to the rest of the universe. It's kind of like an infinitely large Where's Waldo? drawing.
with my head by the fireplace of a stone farmhouse that had stood empty since some time before I was born (20-21)
Now the speaker is looking at his existence in relation to time. In other words, the world was going on long before he was ever part of it. This is the small, insignificant feeling that we get when we think about how big the word (and history) actually is. When you think about it, we are very, very, very itty bitty.
I would not have travelled so far to lie shivering (22)
Travel here seems like a kind of quest. Epic. Maybe our speaker, feeling lost in his life, set out to figure some personal things out, and went on a journey toward self-discovery.
nor have watched the unctuous doctor hold up his needle (24)
This is kind of a creepy line. It's almost as if our speaker is being confronted by his own mortality. His sickness, and the doctor's presence, might be making him think about the meaning of his life more than he normally would.
I would not have seen through the cracked pane the darkening valley with its river sliding past the amber mountains (26-27)
Our speaker gazes out the window, contemplating the beauty of the world and his place in it. Nice.
nor have wakened hearing plums fall in the small hour thinking I knew where I was as I heard them fall (28-29)
Man, it's great to be alive! The speaker seems to be grateful to be able to appreciate the subtle beauty of plums falling at dawn. Having looked back so far and taken notice of the hugeness of the universe, our speaker is happy to be occupying his small speck within it.