You can also use two bolts 1/4x20 or so works fine in about 4" length.

The split pins that hold the pivot to the arm can be gently tapped
out downward using that hammer, a suitable rock or even a cheap
ratshit to drive the bolt into the hole and force out the pin. Once
the pin is out note that one end is slightly tapered. That's the
end you will put back in first. Now pull out the bolt. The armrest
will be free and you can inspect the mounting to the seat frame and
ensure the fasteners are secure. A bit of fingernail polish, your
choice of color will lock the fasteners enough to prevent them from
loosening.

Do whatever you had in mind to the arms, then to reassemble, align
holes as best you can and insert one of the bolts from the bottom
towards the top. Suitable wiggling, tapping and incantations will
get the bolt aligned through the lower hole, the shaft and the upper
hole. Now turn that second bolt around so the head will contact the
pin. Put the tapered end of the pin in the hole, tap gently using
the second bolt head on the pin and the pin should pass through the
assembly easily. The lower bolt will fall out when you're done and
the head of the upper bolt will prevent driving the pin in too
far. How do I know all this? If you sit by the river long enough
a leaf from every tree will float by.