The transaction details -do- track targets killed.
Until you kill your first one, it doesn't show anywhere, but once you get your first kill it adds a line above the 'targets - 5' line, to show how many are killed.
When you get offered the alternate, it doesn't reset your kill count but it does disappear. Once you get your next kill, the 'kills - #' line will reappear and it will show your kills correctly again.
Also, once you have gotten all of your kills, you get a blue triangle popup with the words 'Objective Complete' where you see your other blue popups, like bounties obtained and the like.
then the mission doesnt want "traders". It wants specific traders of another faction. It needs to start specifying these details in the mission briefing. Flying around just killing any trader vehicle is insane. Due to the unknown one could kill 10 "traders" and none would count. Do you not see an issue here?