Lines 19-29 Summary

Get out the microscope, because we’re going through this poem line-by-line.

I would not have found myself on an iron cot
with my head by the fireplace of a stone farmhouse

Finally! We get the second part of the "if" clause. Woo-hoo! Hopefully this "would not have" statement will extend long enough to explain all of the "ifs" that have been introduced so far.
Our speaker's back! More specifically, he's in some old-timey stone farmhouse. It sounds as though he didn't plan to be there ("I found myself"), but there he is.
We're getting excited that this speaker's letting himself into this poem little by little.

that had stood empty since some time before I was born

The farmhouse our speaker is in was uninhabited for a long time. Could it be a relative's farmhouse?
We're getting a good sense of time. It's hard to tie all of this information that predates our speaker together, but this line helps somewhat.
Things are getting interesting. Why is our speaker lying in this old farmhouse? Is he visiting? Has he inherited it? Is anyone else there? C'mon, buddy, dish!
We're on the edge of our seats.

I would not have travelled so far to lie shivering
with fever though I was wrapped in everything in the house

Our speaker is sick, which is why he's lying on the cot. Bummer.
And he doesn't live there permanently. In fact, it seems like he lives pretty far away.
This is the second mention of sickness in the poem. The first was the soldier's friend's older brother's sickly son.
Maybe the speaker has so much time for strange and intricate reflection about how he got where he is (both literally in this farmhouse, and his very existence) because he's been lying on a cot with a fever.

nor have watched the unctuous doctor hold up his needle
at the window in the rain light of October

"Unctuous" has a lot of definitions. One definition is oily or greasy. We don't think this is what he means, though. (Come on, an oily doctor?) Another definition, though, is smug or ingratiating, which means trying to flatter people too much. In other words, this doctor is kind of wormy and smarmy. Maybe the doctor is trying to use flattery to distract the speaker before he jabs him with the needle!
The speaker is sick enough to have a doctor come to him to either inject medication or draw blood. Not fun.
Merwin contrasts the somewhat spooky picture of the doctor holding up a needle with the much prettier description of the how the light looks out the window in October after it's been raining.

I would not have seen through the cracked pane the darkening
valley with its river sliding past the amber mountains

The cracked window is evidence of the farmhouse having been abandoned for so long.
Even though pane and pain have entirely different meanings, they're homonyms (they sound the same), and Merwin might have wanted us to keep the pain of our sick and suffering speaker in mind when he chose that word.
More beauty! As the speaker looks out the window he sees this beautiful, if a little damp, natural view.
"I would not" is the final resolution of all of those previously mentioned "ifs." So if he had not met the red-haired boy—and on and on and on—then he would not be here right now looking at this beautiful view. All of these "ifs" seem to be coming together on this sickbed in the farmhouse.

nor have wakened hearing plums fall in the small hour

The phrase "small hour" is kind of like "wee hours"—in other words, early.
So the speaker would not have been able to see the beautiful sights out the window, or heard the peaceful sound of plums falling, if all the things he mentioned before hadn't happened.
So at first these things seemed unrelated—or at least, distant from our speaker. But really, it seems like even seemingly unrelated events had to fall into place in order for our speaker to end up where he is now. Maybe there's some kind of causal force out there that is beyond our understanding.
In terms of scale—big things to small—these plums are pretty interesting. Merwin has spent most of the poem explaining major events in people's lives—dying in a war, getting sick, moving to a big city—but he's closing the poem with this really simple and soft sound of plums falling. Though things were twisting and turning for most of the poem, it seems like they're quieting down at the end.

thinking I knew where I was as I heard them fall

Hmm. This line is working on a couple of different levels.
First, you know that feeling when you wake up in a strange bed and can't quite figure out where you are for a second? That could be the feeling Merwin is describing here, on a literal level.
On a deeper level, though, maybe the speaker is talking more about an overall sense of identity than simply a physical place. Think about it: he's spent the whole poem tracing back all these events in different parts of the world that led to this moment at the farmhouse. The other people and their histories help create the speaker's sense of where he is in his life.
At least, they give him that overall impression. It's important to note there that he thinks he knows where he is, but he doesn't say that he knows for sure.
Some uncertainty still remains, even in this peaceful moment.
In any case, though, the poem, which has spent so much time and energy reporting the factual events of other people's lives, ends on a quiet, personal note of reflection.