If I had not met the red-haired boy whose father (1)
If the speaker had never met this boy, he wouldn't be where (and who) he is today. Our speaker didn't choose to meet this boy, he just did.
and if I had written anything else at the top
of the examination where it said college (10-11)
This could go either way, but it seems like more of a choice, or free will moment. The speaker chooses a certain college and that sends his life down a certain path.
[…] or if the questions that day had been put differently […] (12-13)
Now Merwin is mixing it up. The speaker chose the college, but not the difficulty of the questions. So there's a little mix of a fate and free will here.
[…] if a young woman in Kittanning had not taught my father to drive at the age of twenty so that he got the job with the pastor at the big church in Pittsburg where my mother was working […] (13-16)
The meeting of his parents seems like pure fate. (Aww.) Outside forces drew them to Pittsburgh where they met.
[…] and if my mother had not lost both parents when she was a child so that she had to go to her grandmother's in Pittsburgh (16-18)
This dash of fate applies both to how the mother and father met, and to how the speaker ends up in that farmhouse. Mysterious and unpredictable forces brought them together, and put the speaker where he is. Thanks tons, forces!