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 beenput 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.