Spread the cotter pin legs apart then hold them together with your pliers to push it through the pin hole. It'll be tight enough in the pin hole that you can take your time getting it spread. I prefer duckbill pliers to work with cotter pins, more control than needle nose.
Checking the service manual a pic of the subject pins shows the head of the pins on the left side of the linkage. That is the opposite side that Loren and I both indicated they should be inserted. Service manual pics are not always correct but my pea brain is too fuzzy to remember the rule or even figure out which way they should be inserted.
In your other post you indicated that the clutch worked ok. Did the clutch engage and disengage or was it engaged only?
Your 2 link pieces are broken on opposite ends like a rotational fracture. Pic1 is that a piece of a link under the head of the pin? Are your other 4 links still in place? Does your sliding collar move freely in the slack of the links?