Does this mean you can do a second task during the legion mission after you complete one of the others?

Nope.

Optional means it will not contribute to Legion Mission success rate. (I bet this means you can leave legion after you got your reward)

You would lose out on the legion success reward then if there is one for the given mission.

Yup, but that will mostly relics, while some traders sometime is already wanting to come back to their original legion