User mark tasks as being completed and they go grey, however when I reopen the plan they appear as still being live. I am losing control and visibility of the tasks completed.
You are using a saved plan copy which does not have global functionality. Actuals are only recorded from the Current Plan. So, when you complete tasks from using the Actuals tab within Edit Tasks, they will only stay completed from the Current Plan and not from any of the plan copies, within Saved Plans of Plan Explorer .