Is it possible to a simple user, view his completed tasks? Even if is just for the non archived processes?
Once a Task is completed it is rolled into the overall instance which moves forward. The task no longer exists as a separate entity that the end user can review. You could build into your process functionality to save this info, for example an email could be sent to the end user with their specific choices, or this info could be written into another location such as into a ticket in service desk.
Retrieving data ...