It's not so much in the words that Merwin's having fun here, but in the syntax, or order, of his lines and phrases. Syntax isn't usually something we give a lot of conscious thought to. Those of us who are native English speakers know that there's a natural pattern to our language, and we form sentences based on that unconsciously.
Inversion, for example, sounds kinda weird to us. We wouldn't say "red the sweater is" (unless we were wearing a Yoda mask). Nope, we'd say "the sweater is red." Both are grammatically correct, but one sounds much more natural. So our brain has built-in expectations about the order of sentences. In the case of this poem, when we read the word "if," our brains expect a two-part sentence: the conditional (the "if" part), and the resolution (a "then" part).
For example: If I win the lottery, then I will buy a lifetime's supply of Almond Joys. After the "if," our brains are wired to expect a "then." What sneaky little Merwin does, though, is delay the "then." He keeps us in expectation to make us curious enough to read on before he offers any resolution. In the first line he says, "If I had not met the red-haired boy…" but, instead of giving us the resolution, he keeps on delaying it. He introduces a whole bunch of other "ifs" until finally, in the nineteenth line (!), he begins to give us a hint of resolution with, "I would not have found myself on an iron cot." Boy howdy, this guy really knows how to create tension.
Another way to keep us coming back, besides manipulating our expectations of syntax, is with anaphora, which is the repetition of a word or phrase. Merwin skates us through the poem with "if this" and "if that" until we get to the "I would not this" and "I would not that." It's a way of both moving the poem forward and keeping it unified. It would be pretty easy to get lost in a poem like this with so much information and no punctuation, so Merwin uses these little tricks of language to keep us on track.